Anti-reflective and anti-soiling coatings with self-cleaning properties

ABSTRACT

Disclosed herein is a method of forming a glass coating including making a sol by hydrolyzing an organosilane in the presence of a least one solvent and at least one catalyst, further adding at least one alkoxysilane, and aging the sol for at least 24 hours.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of the following U.S. patentapplication, which is incorporated by reference in its entirety: U.S.application Ser. No. 14/852,872, filed Sep. 14, 2015(ENKI-0010-U01-001).

U.S. application Ser. No. 14/852,872 (ENKI-0010-U01-001) is acontinuation of the following U.S. patent application, which isincorporated by reference in its entirety: U.S. application Ser. No.14/488,923, filed Sep. 17, 2014, which issued on May 31, 2016 as U.S.Pat. No. 9,353,368 (ENKI-0010-U01).

U.S. application Ser. No. 14/488,923 (ENKI-0010-U01) is acontinuation-in-part of the following U.S. patent application, which isincorporated by reference in its entirety: U.S. application Ser. No.13/184,568, filed Jul. 18, 2011, which issued on Oct. 21, 2014 as U.S.Pat. No. 8,864,897 (ENKI-0002-U01).

U.S. application Ser. No. 13/184,568 (ENKI-0002-U01) is acontinuation-in-part of U.S. application Ser. No. 12/769,580, filed Apr.28, 2010 (ENKI-0001-U01), which claims the benefit of U.S. ProvisionalApplication No. 61/174,430, filed Apr. 30, 2009 (ENKI-0001-P01).

The entirety of each of the foregoing applications is incorporated byreference herein.

STATEMENT REGARDING GOVERNMENT FUNDING

This invention was made with government support under ContractDE-EE0006040 awarded by the U.S. Department of Energy. The governmenthas certain rights in the invention.

BACKGROUND

Field

The embodiments of the disclosure are directed to coatings and theiruses. More particularly, the embodiments of the disclosure are directedto coating compositions that include silane-based precursors that areused to form coatings through a sol-gel process. The resulting coatingsare characterized by anti-reflective, abrasion resistant, andanti-soiling properties. The coatings also have extended weatherabilityto heat and humidity and protection against ambient corrosives. Thecoatings formed from the compositions described herein have wideapplication, including, for example, use as coatings on the outer glassof solar cells or panels.

Description of Related Art

Anti-reflective coatings are used in a wide variety of commercialapplications ranging from sunglasses, windows, car windshields, cameralenses, solar panels, and architectural systems. These coatings minimizethe reflections on the surface of the glass as the light rays travelthrough a discontinuous dielectric gradient. The reflection of lightusually results in reduced transmittance of the light across thetransparent material. For optical applications, it is important that amajority of incident light passes through the interface for maximumefficiency. In this context, anti-reflective coatings provide a usefulbenefit in optical applications.

Anti-reflective coatings are normally used in glasses, acrylics, andother transparent materials that serve as windows and glass panelsassociated with architectural structures or energy generating and savingsystems. In building windows, they are used to maximize influx ofincident light to maintain proper lighting or natural ambience as wellas to minimize distracting reflections from glass surfaces. In energygenerating and saving devices, such as solar panels and lightcollectors, the utility of anti-reflective coatings lies in the enhancedefficiency of these devices due to a greater degree of lighttransmittance and, therefore, increased energy generation for the samecost.

In order for optical elements to perform optimally, it is necessary thatthey be free from surface contamination and depositions (e.g., dirt)that may reduce light transmittance and, therefore, performance of thecoatings. In particular, for optical elements that are exposed to anoutside environment, such as solar panels and building windows, the longterm exposure to chemical and physical elements in the environmentusually results in deposition of dirt on the surface of the opticalelement. The dirt may comprise particles of sand, soil, soot, clay,geological mineral particulates, air-borne aerosols, and organicparticles such as pollen, cellular debris, biological and plant-basedparticulate waste matter, and particulate condensates present in theair. Over time, deposition of such dirt significantly reduces theoptical transparency of the optical element. As a result, there isconsiderable expenditure of human and financial resources associatedwith regular cleaning of such optical elements, such as transparentwindows and solar panels.

The deposition of dirt on such optical elements can be classified intotwo types: physically bound and chemically bound particulate matter. Thephysically bound particles are loosely held due to weak physicalinteractions such as physical entanglement, crevice entrapment, andentrapment of particulates within the nanoscale or microscale edges,steps, terraces, balconies, and boundaries on the uneven surface of theoptical element, such as a window surface. These particles can bedislodged with moderate energy forces such as wind, air from amechanical blower, or by means of water flow induced by rain or otherartificially generated sources of flowing water such as a water hose orsprayer. On the other hand, chemically bound particles are characterizedby the presence of chemical interactions between the particlesthemselves and between the particulate matter and the optical elementitself, such as glass or acrylics (e.g., plexiglass) used, for example,in windows. In these cases, removal of these particles becomes difficultand usually requires the use of physical means such as high pressurewater hoses or manual scrubbing or both. Alternatively, chemical meanssuch as the application of harsh solvents, surfactants, or detergents tothe optical element to free the dirt particles from the surfaces can beused.

As noted, the dirt on ambiently exposed optical elements, such aswindows and solar panels, may be somewhat removed based upon naturalcleaning phenomenon such as rain. However, rain water is only effectiveat removing loosely (physically) held particulate matter and is not ableto remove the particulate matter that may be strongly (chemically)bonded to optical element, such as the glass or window surfaces.Furthermore, rain water usually contains dissolved matter that isabsorbed from the environment during its descent that can leave avisible film when dried.

As such, all externally exposed optical elements, such as windowmaterials and solar panels in which the optimal transmission of light isimportant, require some form of routine cleaning efforts associated withtheir maintenance regimen. In fact, the surfaces of these items arecleaned during fabrication as well as routinely during use. The surfacesof these items, such as solar panels, are usually cleaned with water,detergent, or other industrial cleaners. As a result, anti-reflectivecoating materials applied to these optical elements need to be able towithstand the use of normal cleaning agents including detergents, acid,bases, solvents, surfactants, and other abrasives to maintain theiranti-reflective effect. Abrasion of these coatings over time due tocleaning and the deposition of dirt or other environmental particulatemay reduce their performance. Therefore, abrasion resistance is animportant consideration for anti-reflective coatings. For example,resistance to abrasion is an important consideration for a coating usedin connection with a solar panel, particularly for long term functionalperformance of the solar panel.

A majority of anti-reflective coatings are based on oxides as preferredmaterials. Some anti-reflective coatings are made of either a veryporous oxide-based coating or, alternatively, are comprised of stacks ofdifferent oxides. These oxide materials are chemically reactive withdirt particles by means of hydrogen bonding, electrostatic, and/orcovalent interactions depending upon the type of coating material andthe dirt particle. Therefore, these oxide based coatings have a naturalaffinity to bind molecules on their surfaces. Further, highly porouscoatings can physically trap dirt nanoparticles in their porousstructure. As a result, current anti-reflective coatings arecharacterized by an intrinsic affinity for physical and/or chemicalinteractions with dirt nanoparticles and other chemicals in theenvironment and suffer from severe disadvantages in maintaining a cleansurface during their functional lifetime.

Further, one of most common issue frequently associated withanti-reflective coatings is their performance over the entire solarspectrum, particularly with respect to solar panels. While there areseveral anti-reflective coatings that are only effective in a narrowregion of the solar spectrum, for maximum efficiency it is desirablethat anti-reflective coatings perform equally well over the entire solarregion from 300 nm to 1100 nm.

Consequently, there exists a need in the art for a coating that canprovide the combined benefits of anti-reflective properties, such as acoating that can reduce light reflection and scattering from theapplicable optical surface; anti-soiling or self-cleaning properties,such as a coating surface that is resistant to binding and adsorption ofdirt particles (e.g., resistant to chemical and physical bonding of dirtparticles); abrasion resistant properties, such as stability againstnormal cleaning agents such as detergents, solvents, surfactants, andother chemical and physical abrasives; and UV stability or suitableperformance over the entire solar region.

Further, it would be beneficial for such coatings to be mechanicallyrobust by exhibiting strength, abrasion resistance, and hardnesssufficient to withstand the impact of physical objects in theenvironment such as sand, pebbles, leaves, branches, and other naturallyoccurring objects. It would be beneficial for such coatings to alsoexhibit mechanical stability such that newly manufactured coatings orfilms would be less likely to develop cracks and scratches that limittheir optimum performance, thereby allowing such coatings to be moreeffective for a relatively longer term of usage. In addition, it wouldbe beneficial for such coatings to be able to withstand otherenvironmental factors or conditions such as heat and humidity and to bechemically non-interactive or inert with respect to gases and othermolecules present in the environment, and non-reactive to light, water,acid, bases, and salts. In other words, it is desirable to providecoatings having a chemical structure that reduces the interaction of thecoating with exogenous particles (e.g., dirt) to improve the long termperformance of the coating.

It would also be preferable to enable deposition of such coatings ontothe optical surface, such as the surface of a window or solar panelsurface, using common techniques such as spin-coating; dip-coating;flow-coating; spray-coating; aerosol deposition; ultrasound, heat, orelectrical deposition means; micro-deposition techniques such asink-jet, spay-jet, or xerography; or commercial printing techniques suchas silk printing, dot matrix printing, etc.

It would also be preferable to enable drying and curing of such coatingsat relatively low temperatures, such as below 150° C. so that thecoatings could be applied and dried and cured on substrates to whichother temperature sensitive materials had been previously attached, forexample a fully assembled solar panel.

SUMMARY OF THE INVENTION

The present disclosure provides coating compositions comprising a silaneprecursor or combination of silane precursors, a solvent, optionally anacid or base catalyst, and optionally another additive. The coatingcompositions are hydrolyzed to provide a sol that can be coated on asubstrate from which a gel is formed that is subsequently dried andcured to form a coating having a combination of anti-reflectiveproperties, anti-soiling or self-cleaning properties, and abrasionresistance. Accordingly, the anti-reflective coatings provided by thepresent disclosure are physically, mechanically, structurally, andchemically stable.

In some embodiments, the coating compositions include a combination ofsols containing tetraalkoxysilane, organosilane, and organofluorosilanethat can be used to form coatings for transparent substrates such assolar panels. In some embodiments, the composition of the coatingcomposition is based upon a precise selection of solvent, pH, solvent towater ratio, and solvent to silane ratio that allows the resulting solto remain stable for a significant period of time without exhibitingchange in its chemical or physical characteristics.

The disclosure also provides methods for applying the coatings of thepresent disclosure and for using such coatings. In some embodiments, themethods of treating a substrate comprises pre-treatment of the substratebased on combination of chemical treatment, etching, and/or polishing orcleaning steps that enable a uniform spreading of the sol for making athin film or coating with thickness ranging from 50 nm to 200 nm.Thereafter, in some embodiments, the methods include applying the sol tothe substrate and allowing the sol to gel to form the coating with thedesired properties. In some embodiments, the application of the sol tothe substrate includes drop rolling and/or flow coating that results inuniform deposition of the sol to form an even, uniform and crack-freecoating. In some embodiments, the method includes thermally treating thecoated articles under specific condition of heat and humidity to form achemically durable coating that adheres strongly to the substratewithout cracking and/or peeling.

One aspect of the disclosure is an article. The article includes a glassand a dried gel coating on at least one surface of the glass, thecoating formed from acid-hydrolyzed alkoxysilane, acid-hydrolyzedorganosilane and an acid-hydrolyzed organofluorosilane, wherein thecoated glass has at least one of an anti-reflective property, a highabrasion resistance property, a hydrophobic property, an oleophobicproperty and an anti-soiling property, wherein fluorine is presentthroughout a thickness of the coating.

Another aspect of the disclosure is a solar module coating. The coatingincludes a persistent hydrophobic and oleophobic property, ananti-reflective property that increases solar module efficiency, and ananti-soiling property that reduces energy generation losses due tosoiling and reduces the frequency of washing when compared to equivalentsolar modules without a coating, wherein the solar module coating isselected to reduce the levelized cost of energy of a solar energygenerating system.

In some embodiments, the disclosure provides for the use of the coatingcompositions as an efficiency enhancement aid in architectural windowsin building and houses by the provision of anti-reflection benefitsand/or by the provision of anti-soiling benefits to augment theanti-reflection benefits. In other embodiments, the disclosure providesfor the use of the coating compositions as an efficiency enhancement aidin treatment of transparent surfaces (that require regular cleaning) tomake them self-cleaning.

In some embodiments, the disclosure provides a coated glass-basedarticle suitable for use as outer cover of a solar module assembly thatis anti-reflective, hydrophobic and/or oleophobic and exhibitsresistance to abrasion, uv light, heat, humidity, corrosives such asacids, bases, salts, and cleaning agents such as detergents,surfactants, solvents and other abrasives.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the UV-vis transmittance spectra showing maximumtransmittance enhancement of 2% with coatings on glass slides made fromcomposition given in Example 1.

FIG. 2 illustrates the UV-vis transmittance spectra showing maximumtransmittance enhancement of 3.4% with coatings on a glass slidessubstrate made from composition given in Example 2.

FIG. 3a is an SEM cross-sectional view of a coating made from thecomposition of Example 1 on a glass slide substrate.

FIG. 3b is a SEM oblique view of a coating made from the composition ofExample 1 on a glass substrate.

FIG. 4a is an SEM cross-sectional view of a coating made from thecomposition of Example 2 on a glass slide substrate.

FIG. 4b is a SEM oblique view of a coating made from the composition ofExample 1 on a glass substrate.

FIG. 5 shows an XPS spectrum of a coating made from the composition ofExample 1.

FIG. 6 shows an XPS spectrum of a coating made from the composition ofExample 2.

FIG. 7a illustrates nano-indentation data showing the indenter depthprofile and hardness, of a coating made from the composition of Example1 on a glass slide substrate.

FIG. 7b illustrates nano-indentation data showing the indenter depthprofile and Young's Modulus, of a coating made from the composition ofExample 1 on a glass slide substrate.

FIG. 8a illustrates nano-indentation data showing the indenter depthprofile and hardness, of a coating made from the composition of Example2 on a glass slide substrate.

FIG. 8b illustrates nano-indentation data showing the indenter depthprofile and Young's Modulus, of a coating made from the composition ofExample 2 on a glass slide substrate.

FIG. 9 illustrates the results of accelerated soiling studies on lighttransmission for coatings made according to some embodiments of thepresent disclosure.

FIG. 10 illustrates the results of accelerated soiling studies on dirtadhesion for coatings made according to some embodiments of the presentdisclosure.

FIG. 11 shows an XPS cross section through a coating from thecomposition of Example 1 showing the elemental composition of thecoating in vertical cross-section.

FIG. 12 shows an XPS cross section through a commercially availablehydrophobic anti-reflective coating showing the elemental composition ofthe coating in vertical cross-section.

FIG. 13 shows an SEM oblique view and cross-section of a commerciallyavailable hydrophobic anti-reflective coating.

FIG. 14 shows changes in atomic concentration of fluorine during 3000hours of accelerated environmental stress testing derived from XPSanalysis of two samples made according to the embodiments of thedisclosure.

DETAILED DESCRIPTION

Various embodiments of the disclosure are described below in conjunctionwith the Figures; however, this description should not be viewed aslimiting the scope of the present disclosure. Rather, it should beconsidered as exemplary of various embodiments that fall within thescope of the present disclosure as defined by the claims. Further, itshould also be appreciated that references to “the disclosure” or “thepresent disclosure” should not be construed as meaning that thedescription is directed to only one embodiment or that every embodimentmust contain a given feature described in connection with a particularembodiment or described in connection with the use of such phrases. Infact, various embodiments with common and differing features aredescribed herein.

The present disclosure is generally directed to coatings that provide acombination of benefits including anti-reflective properties,anti-soiling properties, self-cleaning properties and manufacturingflexibility as well as other benefits. Accordingly, the coatings of thepresent disclosure may be used on substrates, such as transparentsubstrates, to increase the light transmittance through the substrates.In particular, the coatings may be used on transparent substrates suchas glass or the front cover glass of solar panels.

The present disclosure is particularly well suited for use with glassused in solar energy generation (“solar glass”). It should be understoodthat solar energy generation includes solar photovoltaic and solarthermal, wherein solar insolation is used to produce heat either as anend-point or as an intermediate step to generate electricity.Furthermore it should be understood that solar glass may be used in anyapplication where maximal transmission of solar energy through the glassis desired such as for example in greenhouses. Typically solar glass ishigh transmission low iron glass. It may be either float glass, that is,flat glass sheets formed on a molten tin bath or rolled glass whereinthe flat glass is formed by the action of rollers. Float glass is oftencharacterized by the presence of tin contamination on the bottom (“tinside”) of the glass. Rolled glass is typically textured on one side toimprove its performance in solar panels. The present disclosure may alsobe applied to glass surfaces used as mirrors in solar energy generationsuch as parabolic trough systems or in heliostats. It may also be usedto coat various glass lenses such as Fresnel lenses used in solarthermal generation. Additionally, solar glass may have various coatingsapplied. For example a common coating is a transparent conduction oxide(CTO) such as indium tin oxide (ITO) or fluorine doped tin oxide on oneside of the glass. This coating is used to provide the front electrodefor many thin film solar panel technologies. Other coatings may bepresent such as coatings to seal in alkali ions such as Na⁺ and Ca⁺ thatare used in the manufacturer of the glass but that cause long termreliability problems when leached out by water. Other techniques tosolve this problem are to deplete these ions in thin layers of the glasssurface. Solar glass may also be coated with a reflective surface toform a mirror. Solar glass may be tempered or untempered. Tempered glassis significantly stronger and solar panels manufactured using ittypically only need one sheet of glass. Solar panels manufactured withuntempered front glass typically need a back sheet of tempered glass tomeet strength and safety requirements. Many thin-film solar photovoltaictechnologies also use the front glass as a substrate upon which theydeposit materials that comprise the solar cell. The processes usedduring the manufacturer of the solar cell may adversely affect theproperties of any existing coatings on the glass or existing coatingsmay interfere with the solar cell manufacturing process. The presentdisclosure is completely tolerant of type of glass selected by the solarpanel manufacturer. It works equally well on float or rolled glass. Itis not adversely affected by the presence tin contamination on floatglass.

One critical issue for solar panel manufacturers that use CTO or ITO (orsimilar) coated glass is tempering. It is very difficult to achievelow-cost, high quality CTO or ITO coated tempered glass. Therefore solarpanel manufacturers that require CTO or ITO coated glass use untemperedglass, necessitating the use of a second sheet of tempered glass on theback side of the solar panel. Additionally even if suitable CTO or ITOcoated tempered glass was available some thin-film solar manufacturingprocesses heat the glass during manufacturer to the extent that thetemper is lost. All existing anti-reflective glass on the market istempered, because the anti-reflective coating is actually formed duringthe tempering process. Tempering is the process by which the glass isheated to 600° C. to 700° C. then quickly cooled. This high temperingtemperature effectively sinters the anti-reflective coating providing itwith its final mechanical strength. Thus solar panel manufacturers thatcannot use tempered glass typically cannot use anti-reflective glass.The present disclosure may be applied and cured at a low temperature ofbetween 20° C. and 200° C. and between 20° C. and 130° C. and furtherbetween 80° C. and 200° C. This low temperature facilitates the coatingof completed solar panels without damage to the panel. Thus it is ananti-reflective solution for users of untempered solar glass.

The low temperature curing of the present disclosure also providessubstantial benefits to solar panel manufacturers beyond enablinguntempered anti-reflective glass. By making possible the coating of theglass without the need for the tempering step, solar panel manufacturersare enabled to apply their own anti-reflective coating. Currently therequirement for a large tempering oven means that solar panelsmanufacturers are restricted to buying anti-reflective glass from glassmanufacturers. This means that they must maintain inventory of bothanti-reflective coated and non-coated glass. As these cannot be usedinterchangeably inventory flexibility is reduced necessitating keepinglarger amounts of inventory on hand. The ability for the solar panelmanufacturer to apply their own coating means that they can just hold asmaller inventory of non-coated glass and then apply the anti-reflectivecoating to that as needed.

In addition, existing anti-reflective coatings are prone to scratchingduring the solar panel manufacturing process. Typically solar panelmanufacturers must use a plastic or paper sheet to protect the coating.As the coating of the present disclosure can be applied to fullymanufactured solar panels, it can be applied at the end of themanufacturing process thus removing the need for the protection sheetand the opportunity for damage to the coating during manufacture.

Existing anti-reflective coatings from different manufacturers tend tohave subtle color, texture and optical differences. This presentsproblems to solar panel manufacturers who desire their products to havea completely consistent cosmetic finish. If they manufacturer largenumbers of solar panels it is almost inevitable that they will have toorder anti-reflective glass from different suppliers causing slightdifferences in the appearance of the final products. However, thecoating of the present disclosure enables solar panel manufacturers toapply their own coating and so enables cosmetic consistency over anunlimited number of solar panels.

In addition, to their anti-reflective properties, the coatings describedherein exhibit anti-soiling and/or self-cleaning properties, as they areresistant to the adhesion of dirt and promote the removal of any adhereddirt by the action of water. More specifically, the coatings describedherein are characterized by extremely fine porosity that minimizes thedeposition of dirt by physical means. Further, these coatings arecharacterized by a low energy surface that resists chemical and physicalinteractions and makes it easy to dislodge the particles, thereby makingthe surfaces essentially anti-soiling. The reduced physical and/orchemical interactions with the environment, such as dirt, make theexposed surface of these coating less susceptible to binding of dirt andalso make it easier to clean with a minimal expenditure of force orenergy.

Typically in order to completely clean ordinary glass a mechanicalaction for example brushes or high pressure jet is required to dislodgedirt that is strongly adhered to the surface. However the coating of thepresent disclosure presents a surface such that dirt is much moreattracted to water then to the surface. Thus in the presence of waterany dirt resting on the surface is efficiently removed without the needfor mechanical action. This means that coated glass will achieve a highlevel of cleanliness in the presence of natural or artificial rainwithout human or mechanical intervention. In addition, the amount ofwater required to clean the glass is substantially reduced. This is ofspecial significance given that the most effective locations for solarenergy generation tend to be sunny warm and arid. Thus water is aparticularly expensive and scarce resource in the very locations thatsolar energy is most effective.

The present disclosure enables a significant reduction in the LevelizedCost of Energy (LCOE) to the operator of a solar energy generatingsystem. First, the anti-reflective property increases the efficiency ofthe solar panels. Increased efficiency enables a reduction of cost inthe Balance Of System (BOS) costs in construction of the solar energygeneration system. Thus for a given size of system the capital costs andconstruction labor costs are lower. Second, the anti-soiling propertyincreases the energy output of the solar panels by reducing the lossesdue to soiling. Third the Operating and Maintenance (O&M) costs arereduced because fewer or no washings are needed eliminating labor andwater cost associated with washing.

The coatings described herein also contain water and oil resistanthydro/fluorocarbon groups that make them chemically non-reactive andnon-interacting. When used in combination with a glass substrate, thecoatings bind to the glass surface using siloxane linkages that makethem adhere strongly and makes them strong, durable, and abrasion andscratch resistant. In summary, these coatings are physically andchemically nonreactive, mechanically and structurally stable,hydrophobic, oleophobic, and stable across the UV spectrum. Accordingly,it should be appreciated that the coatings described herein haveparticular application to transparent substrates that are exposed to theenvironment, such as exterior windows and the front cover glass of solarpanels.

Generally, the coatings described herein are prepared by a sol-gelprocess. The starting composition, referred to as a “coating mixture” or“coating composition,” includes a silane precursor or a combination ofsilane precursors that when hydrolyzed and condensed forms a particulatesuspension of particles in a liquid sol. This sol can be coated onto asubstrate using coating techniques known in the art, gelled to form agel, and dried to form a hard layer or coating having the propertiesnoted above. The process of curing the dried gel further hardens it.

Generally, the resulting properties of the coating described above areprovided by using a particular combination of components in theformation of the final coating. In particular, the selection of aparticular silane precursor or mixture of silane precursors incombination with other components in the coating mixture is important inproviding a coating with the desired properties. For example, in someembodiments, the coatings are made from a mixture of silane precursorsincluding alkoxysilane, organosilane, and organofluorosilane. In someembodiments, separate coating mixtures or mixtures of silane precursorscan be used to form separate sols that may then be combined to form afinal sol that is applied to a substrate to be coated. Further, a singlesol, or separately prepared sols that are combined together, may becombined with another silane precursor to form a final sol that isapplied to a substrate to be coated.

It should be appreciated that the coating mixture may include othercomponents in addition to any silane precursors. For example, the silaneprecursors may be mixed with water or a solvent mixed with water, anacid or base catalyst, and one or more composition modifying additivesto impart, provide, modulate, and/or regulate the intrinsic structures,properties, function, and performance of the resulting coatings. Thecomposition modifying additives may include particle size modifyingadditives, cross-linking additives, and surface modifying additives.Each of these components in the coating mixture is described in moredetail below.

It should be appreciated that long term sol stability—as defined by theretention of liquid state without gelation or precipitation—is importantin practical applications. Accordingly, the sols provided by the presentdisclosure are chemically and physically stable under ambient storageconditions for periods ranging from about 3 months to about 6 months. Insome embodiments, the sol is stable for periods ranging from about 6months to about 9 months when stored at 4° C. The stability of the solis due to several factors including the specific combination of thesilane precursors, control of the pH of the sol, selection of a solventwith a balance of hydrogen bonding, polar, and nonpolar groups, balanceof the solvent to water ratio in the coating mixture, and balance of thesilane(s) to solvent ratio, each of which is described in more detailbelow.

Each of the specific components used in the coating mixture will now bedescribed. As noted, a silane precursor or a combination of silaneprecursors is used in the coating mixture to generate the coatings ofthe present disclosure. The silane precursor used to make the coatingmay be selected based upon the properties desired to be imparted to theresulting coating. In some embodiments, the silane precursor is selectedbased upon its tendency to adhere to glass due to formation of strongSi—O—Si bonds between the surface of the glass and the components of thecoating composition.

For example, tetraalkoxysilanes when hydrolyzed form an extensivelycross-linked structure due to the formation of four Si—O—Si linkagesaround each silicon atom. These structures are characterized bymechanical stability and abrasion resistance. To impart hydrophobicityto the ultimate coating, organically-modified silanes (such asmethyltrimethoxysilane) can be used in addition to thetetraalkoxysilane. Further, to impart oleophobicity and anti-soilingcharacteristics, organofluorosilanes can be used in addition to thetetralkoxysilane.

In some embodiments, the silane precursor can be alkyltrialkoxysilane,tetraalkoxysilane, an organosilane, an organofluorosilane, or acombination of any one or more of these. The alkyl group foralkyltrialkoxysilane can be methyl, ethyl, or propyl. Thetetraalkoxysilane is of the type (OR)₄Si where R=H, methyl, ethyl,isopropyl, or t-butyl. The tretraalkoxysilane can be a methoxy, ethoxy,or isopropoxy or t-butoxy analog. The organosilane is of the type(OR)₃Si—R′ where R=H, methyl, ethyl, isopropyl, or t-butyl, andR′=methyl, ethyl, or propyl. The organosilane precursormethyltrimethoxysilane can be substituted with triethoxy ortri-isopropoxy derivatives. The organofluorosilane is of the type(OR)₃Si—Rf′ where R=H, methyl, ethyl, isopropyl, or t-butyl, andRf′=3,3,3-trifluoropropyl, or tridecafluoro-1,1,2,2-tetrahydrooctyl. Theorganofluorosilane precursor can be(3,3,3-trifluoropropyl)trimethoxysilane or(tridecafluoro-1,1,2,2-tetrahydrooctyl)trimethoxysilane.

In other embodiments, the coating mixture may comprise organosilicamonomers, oligomers, particles, polymers, and/or gels of organosilicatematerials made from a single component or mixtures of starting materialshaving the general formula (X)_(n)Si(R)_(4-n) and/or(X)_(n)Si—(R)_(4-n)—Si(X)_(n) where X comprises halides, alkoxides,carboxylates, phosphates, sulfates, hydroxides, and/or oxides; n=1, 2,or 3; and R is an alkyl, alkenyl, alkynyl, phenyl, or benzenylhydrocarbon and/or fluorocarbon chain with 1-20 carbon atoms and asolvent to dissolve or disperse the organosilane.

As noted, in some embodiments, combinations of silane precursors can beused in the coating mixture. The ratio of these silane precursors can bevaried independently to fine tune and modify the overall characteristicsof the ultimate coating. As such, the stoichiometric ratio of the silaneprecursors is important for the stability of the sol made from theseprecursors, as well as for the function and performance of films andcoatings made from these sols. For example, the relative ratio of eachof these precursors is important to form a stable cross-linked networkfor mechanical stability and abrasion resistance.

In some embodiments three silane precursors are used. In someembodiments, the three silane precursors used may include analkoxysilane, an organosilane, and an organofluorosilane. In someembodiments, the alkoxysilane may be alkyltrialkoxysilane ortetraalkoxysilane. The alkyl group for alkyltrialkoxysilane can bemethyl, ethyl, or propyl. The tetraalkoxysilane is of the type (OR)₄Siwhere R=H, methyl, ethyl, isopropyl, or t-butyl. The tretraalkoxysilanecan be methoxy, ethoxy, or isopropoxy or t-butoxy analogs. Theorganosilane may be of the type (OR)₃Si—R′ where R=H, methyl, ethyl,isopropyl, or t-butyl, and R′=methyl, ethyl, or propyl. The organosilaneprecursor methyltrimethoxysilane can be substituted with triethoxy ortri-isopropoxy derivatives. The organofluorosilane may be of the type(OR)₃Si—Rf′ where R=H, methyl, ethyl, isopropyl, or t-butyl, andRf′=3,3,3-trifluoropropyl, or tridecafluoro-1,1,2,2-tetrahydrooctyl. Theorganofluorosilane precursor can be(3,3,3-trifluoropropyl)trimethoxysilane or(tridecafluoro-1,1,2,2-tetrahydrooctyl)trimethoxysilane. In someembodiments, the three silane precursors used may includetetramethoxysilane or tetraethoxysilane (as the tetraalkoxysilane),methyltrimethoxysilane (as the organosilane), andtrifluoropropyltrimethoxysilane ortridecafluoro-1,1,2,2-tetrahydrooctyl)trimethoxysilane (as theorganofluorosilane). In some embodiments, the total concentration of allof the silane precursors may be from about 1% by weight to about 10% byweight. In some embodiments, the total concentration of all of thesilane precursors may be from about 1% by weight to about 4% by weight.

Aside from the total concentration of all of the silane precursors used,the relative amount of each is also important. For example, in theembodiment with three silane precursors, the relative amounts of eachthat are added to the coating mixture is based upon several factors,including visual sol homogeneity in the coating mixture, uniformity ofthe film or coating once deposited on the substrate, and the desiredabrasion resistance of the coating. More specifically, the relativeamounts of each of the three silanes are adjusted to fine tune solhomogeneity, adjust film or coating uniformity, and to control abrasionresistance of the final coating. It should be appreciated that therelative amounts of the silanes used are determined regardless ofwhether the silane precursors are added to the same coating mixture orwhether separate coating mixtures are used to generate separate solsthat are then combined to form a final sol that is applied to a givensubstrate. In other words, the relative amounts of the silane precursorsis determined based upon the amount of each silane precursor present inthe final sol that will be applied to a given substrate. Regarding solhomogeneity, if three silane precursors are used, each would behydrolyzed in a solvent medium to form a sol. However, an alkoxysilanesuch as tetraalkoxysilane is polar and hydrophilic while theorganosilane and the organofluorosilane are hydrophobic. As a result,they exhibit differential interaction with each other and with thesolvent matrix. High amounts of the organofunctional (organofunctionalrefers to any trialkyloxysilane with an R group which includes bothorgano and organofluorosilanes) silanes lead to phase separation andaggregation while lower amounts of organofunctional silanes leads tofilms with poor anti-reflective and anti-soiling characteristics.Therefore, using the appropriate relative amounts of each of the silaneprecursors is essential for the desired function and application of thecoating mixture on a substrate made from the sol.

Regarding film or coating uniformity, the final sol is applied to asubstrate followed by gelation and evaporation of the solvent to form asilica-based coating. Due to their differential interactions with thesolvent matrix, the particles made from the different silane precursorsdiffer in their solubility. High amounts of the organofunctionalizedsilanes (organosilane plus organofluorosilane) lead to development ofpatchy opaque films due to their limited solubility and phase separationduring solvent evaporation. On the other hand, high amounts oftetralkoxysilanes lead to formation of films that do not exhibitsufficient anti-reflective or anti-soiling characteristics. Regardingabrasion resistance, the organosilanes and organofluorosilanes each arecapable of forming three Si—O—Si linkages, while the tetraalkoxysilanescan form four Si—O—Si linkages. Therefore, the relative ratio of each ofthese precursors is important in forming a stable cross-linked networkfor mechanical stability and abrasion resistance. High amounts of theorganofunctionalized silanes (organosilane plus organofluorosilane) inthe films result in films that are not sufficiently mechanically stableand that lack sufficient abrasion resistance.

In some embodiments, the relative weight percent ratio of thealkoxysilane, such as tetraalkoxysilane, to the total amount offunctionalized silanes (organosilane plus organofluorosilane) lies inthe range of about 0.2 to about 2. In some embodiments, this ratio is inthe range of about 0.2 to about 1, and in some embodiments, it is in therange of about 0.5 to about 1.

Similarly, the relative ratio of organosilane to organofluorosilane isimportant in obtaining a stable sol that can spread evenly on thesubstrate and that does not result in the precipitation or aggregationthat may reduce optical transmission through the coating, for example,if used in connection with a solar panel cover. The relative amounts oforganosilane and organofluorosilane also determine the hydrophobicityand anti-soiling characteristics. The structural similarities of thesetwo precursors as well as their relative solvation and theirintermolecular interactions control their interactions with each otherrelative to any solvent used in the coating mixture as described below.For example, the chain lengths of the organosilane and fluorosilane areimportant in determining the ratio of these components. In general, ifthe side chains on these precursors get too big, then a high amount ofthese precursors would cause opaque films. Therefore, for precursorswith a large R group on the organosilane, its amount would need to bereduced relative to fluorosilane and alkoxysilane. The same holds truefor fluorosilane as well. In some embodiments, the relative weightpercent ratio of organosilane to organofluorosilane is in the range ofabout 0.5 to about 1.5. In some embodiments, this ratio is in the rangeof about 0.75 to about 1.25, and in some embodiments it is in the rangeof about 0.5 to about 1. Depending upon the use of the ultimately formedcoating, in some embodiments, the amount of organosilane or mixture oforganosilanes can vary typically from about 0.1% to about 90%, fromabout 10% to about 65%, and from about 10% to about 25%, by weight ofthe coating composition or mixture. Examples of organoflurosilanes thatcan be used includetridecafluoro-1,1,2,2-tetrahydrooctyl)triethoxysilane;tridecafluoro-1,1,2,2-tetrahydrooctyl)dimethylchlorosilane;3,3,3-trifluoropropyl)trimethoxysilane;3,3,3-trifluoropropyl)trichlorosilane; and3,3,3-trifluoropropyl)methyldimethoxysilane; Examples of organosilanesthat can be used includemethyltrichlorosilane, methyltrimethoxysilane,phenylmethyldiethoxysilane, dimethyldimethoxysilane, andn-propylmethyldichlorosilane. It should be appreciated that theforegoing fluorosilanes and organosilanes may be used alone or in anycombination.

As noted, in some embodiments, a solvent may be used in the coatingmixture with the silane precursor or combination of silane precursorsdescribed above. For example, a solvent may be used in a coating mixturethat is used to form a single sol, or a solvent may be used inconnection with the formation of separate sols that may be combined toform a final sol for coating a given substrate.

Generally, the solvent is an organic solvent that is in a mixture withwater. The nature and amount of the solvent along with the solvent/waterratio and the solvent/silane precursor ratio used in the coating mixtureis important in the stability of the sol, as well as in imparting thespecific function and performance of the coatings made from the solcontaining the hydrolyzed silane precursors. The choice of solvent alsoaffects the viscosity of the sol or spreadability of the sol on thesubstrate to be coated, as well as the intermolecular interactions ofthe sol particles and their interactions with the substrate surface tobe coated. In addition to providing a matrix for carrying out thesol-gel reaction, the solvent also influences the reaction kinetics ofthe sol formation steps. Each of these effects on the selection of asolvent and the amount of solvent to be used is described in more detailbelow.

One factor is the effect of the solvent on hydrolysis and condensationof the silane precursors. In the sol-gel process the silanes arehydrolyzed to silanols that further condense to form siloxane linkages.The particle sizes and the consequent viscosity of the sol depend uponthe number of particles and the state of their aggregation in the sol. Asolvent that exhibits strong hydrogen bonding and has hydrophilicproperties can stabilize individual sol particles to make the sol lessviscous and more stable for a longer period of time. A less viscous solcan form thin films on the order of 50-200 nm that are beneficial inproviding a coating that provides an anti-reflective effect.

Another factor is the effect of the boiling point and rate ofevaporation of the solvent on formation of a uniform film on thesubstrate from the sol. Solvents with relatively low boiling points and,therefore, having an appreciable rate of evaporation are more desirablein forming a uniform film from the coating mixture or sol. As thesolvent evaporates, the silicate particles come closer together and forma network. If the solvent evaporates too rapidly the films may not beuniform. Similarly, a solvent with high boiling point does not evaporateat an appreciable rate under ambient conditions, thereby leading toformation of less desirable opaque films with larger particles.

The solvent also affects the overall spreadability of the sol during theapplication of the sol on the substrate since the solvent affects thesurface energy of the sol. Solvents with lower surface tension enablemore even spreading of the sol onto the substrate. Accordingly, in someembodiments, the solvent provides a balance of both hydrophobic andhydrophilic groups to stabilize the sol and to provide for an optimumsol viscosity. For example, alcohols with a moderate chain length of 3-6carbon atoms are particularly advantageous in lowering the surfacetension in the sol.

Another factor is the effect of hydrogen bonding and otherintermolecular interactions on the stability of the sol. Solvents thatcan participate in hydrogen bonding interactions stabilize the solparticles by preventing aggregation and precipitation when the sol isdeposited onto a substrate. Solvents containing hydrogen bond donor —OHfunctional groups, such as alcohols and glycols, along with solventscontaining hydrogen bond acceptor R—O—R linkages, such as ethers, areparticularly advantageous in preparation of stable sols.

Another factor is the effect of a solvent's toxicity, volatility, andflammability. The toxicity and flammability of the solvent can be aconcern in the long term storage and transportation of the sol.Therefore, solvents with low toxicity, volatility, and flammability arepreferred. Alcohols, glycols, and glycol ethers are particularlyadvantageous in this regard.

Considering the above factors in the selection of a solvent, in someembodiments, organic solvents that may be used are ones that containhydrogen bond donor groups, such as alcohols and glycols with —OHgroups, and hydrogen bond acceptor groups, such esters (—COOR), ethers(R—O—R), aldehydes (RCHO), and ketones (R₂C═O). Another group ofsolvents that may be used include glycol ethers and glycol ether esters.These solvents provide a balance of the factors outlined above forstability, viscosity, and spreadability of the sol. In some embodiments,the solvent that may be used in the coating mixture includes methanol,ethanol, 1-propanol, 2-propanol (isopropanol), n-butanol, isobutanol,tert-butanol, pentanol, hexanol, cyclohexanol, ethylene glycol,propylene glycol, ethylene glycol monomethyl ether, ethylene glycoldimethyl ether, propylene glycol monomethyl ether, diglyme, acetone,methyl ethyl ketone (butanone), pantanones, methyl t-butyl ether (MTBE),ethyl t-butyl ether (ETBE), acetaldehyde, propionaldehyde, ethylacetate, methyl acetate, ethyl lactate, and any combination of any ofthe foregoing. In other embodiments, the organic solvent may be aketone. In other embodiments, the solvent may be acetone, ethanol,propanol, isopropanol, butanol, t-butanol, etc. It should be appreciatedthat the foregoing solvents may be used alone or in any combination.

As noted, the solvent may be used in combination with water. Therelative ratio of the solvent to water is important to obtain thedesired particle sizes of the sol, which, in turn determines theviscosity of the sol and the resulting thickness of the films formedfrom the sol once applied to a substrate. As noted above, the thicknessof the film is important in providing a coating having suitableanti-reflective properties.

In addition, the relative amount of solvent to water and the solcomposition play a key role in stabilizing sol particles so that theparticles remain effectively suspended in the solvent matrix withoutaggregation. Specifically, a balance of hydrophobic and hydrophilicgroups is necessary to provide a sol that is stable for extendeddurations, which in some embodiments may be more than four months, suchthat the sol does not exhibit large changes in its properties and canstill be spread evenly on the substrate, such as a glass surface. Thisstability is also important in enabling the proper spreading of the solduring its application to a substrate.

Essentially, the solvent to water ratio dictates the balance ofhydrophobic and hydrophilic sites in the matrix of the sol and thesolvent. Hydrolysis of silane precursors forms hydroxylated particlescontaining silanol groups. These silanol groups are effectivelysolubilized by a hydrophilic solvent that provides hydrogen bonding andmay reduce cross-linking resulting from condensation reactions.Condensation of these silanol groups to form non-polar Si—O—Si linkagesresults in cross-linking of the particles, and, due to a reduction insurface hydroxyl groups, the cross-linked structures are energeticallyfavored in a medium that is more hydrophobic. Similarly, thecondensation reaction causes the release of water molecule, which isretarded in a medium with high water content. Conversely, a medium withhigh water content favors hydrolysis of silane precursors to formhydrolyzed sol particles that can combine to form a precipitate.Therefore, the solvent to water ratio is important to form a stable sol.

In particular, the solvent to water ratio plays a significant role insolubilization of the sol particles containing, for example, thosegenerated from silane, organosilane, and organofluorosilane precursors.The nature of the silane precursors dictates a range of functionalgroups on the surface of sol particles that range from hydrophilic andhydrogen bond donating Si—OH groups to polar and hydrogen bond acceptingSi—O—Si groups to nonpolar and hydrophobic organo and fluoroalkylgroups. The solubility and inter-particle interactions depend stronglyupon the composition of the solvent matrix especially, as noted, thehydrophobic and hydrophilic balance of the solvent matrix.

Accordingly, in some embodiments, the solvent to water weight percentratio in the coating mixture that has been found to be most advantageousto the stability and spreadability of the sol is in the range of about 5to about 20. In some embodiments, the solvent to water weight percentratio is in the range of about 6 to about 12, and in some embodiments itis in the range of about 7 to about 10.

The ratio of the combined weight percent of solvent, including in thiscase the water as well, (i.e., organic solvent(s) plus water) to that ofthe combined weight percent of the silane precursor (i.e., the total ofall silane precursors used in the coating mixture if more than onesilane precursor is used, regardless of whether the silane precursorsare added together to form one sol or whether separate sols are formedthat are then combined) plays an important role in the long termstability and functional effectiveness of the coatings made from thecoating mixtures. Specifically, this ratio can affect several aspects ofthe sol, including the sol particle size, viscosity, and stability.

The solvent is used to solubilize the silane precursors as well as toeffectively disperse the sol particles. A reaction medium that containsa high amount of silane precursor has a high effective concentration ofsilanes that accelerate the hydrolysis reaction to form smallerparticles, while a reaction medium containing a lower concentration ofsilane precursor enables formation of larger particles in the sol. Solscontaining very small particles, such as particles less than 10 nm insize, result in transparent films; however, they may not exhibit anyanti-reflective effects. Sols containing larger particles, such asparticles larger than 100 nm in size, produce films that tend to besemi-transparent or opaque. Therefore, in some embodiments, solparticles having a size in the range of 10-50 nm are desired.

The relative ratio of solvent to silane also affects viscosity andsurface tension of the sol. Highly viscous sols with a smaller solventto silane precursor ratio can form coatings that are too thick, whilevery low viscosity sols with a very high solvent to silane precursorratio results in patchy films and coatings. Therefore, the relativeamount of solvent to silane precursors in the sol is important inmaintaining proper consistency of the sol and to provide a sufficientmass of silane to form films and coatings with desired thicknesses. Insome embodiments, the ratio of solvent to silane precursor is controlledto produce coatings having a thickness of about 60 nm to about 150 nm,which is important to provide an anti-reflective effect.

The relative ratio of solvent to silane precursor is also important incontrolling the isolation and separation of sol particles to preventinter-particle aggregation and agglomeration. At high ratios of solventto silane precursor (i.e. low concentrations of silane precursorrelative to the solvent) the particles are effectively dispersed by thesolvent matrix, which keeps them separate and reduces or preventsaggregations. However, the films made from such sols may lack theappropriate thickness to impart useful functional benefits ofanti-reflection, anti-soiling, and abrasion resistance. At low ratios ofsolvent to silane precursor (i.e., high concentration of silanesrelative to solvent), the particles are not effectively solubilized andsuspended in the solvent matrix resulting in aggregation andprecipitation. Coatings made from these sols are opaque or semi-opaqueand generally may not be suitable for use as anti-reflective coatings.

Therefore, it should be appreciated that a precise balance of solvent tosilane precursor is important for the stability of the sol, as well asthe function and performance of the coatings made from sols. In someembodiments, this effective weight percent ratio of total solvent (i.e.,water plus organic solvent) to total silane precursors is in the rangeof about 25 to about 125. In some embodiments, this ratio is in therange of about 40 to about 99, and in some embodiments, this ratio is inthe range of about 50 to about 75. The range for this ratio applies toeach coating mixture used to form a sol that may be combined to form thefinal sol that is applied to a given substrate.

In light of the above considerations and ratios, the amount of solvent,including in this case the water as well, (i.e., organic solvent(s) pluswater) in the overall coating mixture from which the sol is formed canvary from about 50% to about 90% by weight. In some embodiments, theamount of solvent in this mixture may be from about 80% to about 90% byweight. In one embodiment, a mixture of silane precursors including atetraalkoxysilane, an organosilane, and an organofluorosilane may beused with isopropanol as the organic solvent in water. In this case, theamount of isopropanol and water may be from about 80% to about 90% byweight. In one embodiment, a mixture of silane precursors includingtetramethoxysilane or tetraethoxysilane, methyltrimethoxysilane, andtrifluoropropyltrimethoxysilane may be used with isopropanol as theorganic solvent in water, the amount of solvent may be 86.4% by weight.The range for this ratio also applies to each coating mixture used toform a sol that may be combined to form the final sol that is applied toa given substrate.

The silane precursor(s) and solvent may also be mixed with an acid or abase as a catalyst to accelerate hydrolysis of the silane precursors.The nature and amount of the acid or base used is important for thestability of the sol as well as function and performance of the coatingsmade from the sol containing hydrolyzed silane precursors.

When acids are used as catalysts, they can be either a weak acid or astrong acid. They can also be organic acids or inorganic acids. Whenbases are used as catalysts, they can be either a weak base or a strongbase. Similarly, they can also be organic bases or inorganic bases.Acids and bases persist in the sol after the reactions. Examples ofstrong acids include hydrochloric acid and nitric acid. Examples of weakacids include acetic acid, trifluoromethanesulphonic acid, and oxalicacid. Examples of strong bases include potassium hydroxide, sodiumhydroxide, and aqueous ammonia. Examples of weak bases include pyridine,tetraethylammonium hydroxide, and benzyltriethylammonium hydroxide.

Since the acid or base catalyst persists in the sol, their concentrationin the medium controls the final pH of the system. For maximum stabilityof the sol, the pH of the final sol should be controlled to preventaggregation and precipitation, since the sol particles otherwise have atendency to aggregate and from viscous gels or precipitates. Similarly,the pH of the sol has been determined to be important in maintaining aproper range of pH for long term sol stability. At low pH the silaneparticles are positively charged and, therefore, can stay separated dueto inter-particle electrostatic repulsions. At high pH, there is atendency to form anionic silicates that do not form a network.

Two distinct regions of pH stability have been identified that enhancethe long term stability and durability of the sol. With acid catalysts,in some embodiments, the effective pH is in the range of about 1 toabout 4. In some embodiments, this effective pH is in the range of about2 to about 4, and in some embodiments, this effective pH is in the rangeof about 2.5 to about 3.5. With base catalysts, in some embodiments, theeffective pH is in the range of about 7 to about 10. In someembodiments, this effective pH is in the range of about 7 to about 9,and in some embodiments, this effective pH is in the range of about 7.5to about 8.5.

In some embodiments, depending upon the application of the sol, theamount of the catalyst can vary from 0.001% to about 2%, from about 0.1%to about 1%, and from about 0.01% to about 0.1%, by weight of thecoating mixture from which the sol is made. Nonlimiting examples ofuseful catalysts are HCl, HNO₃, H₂SO₄, H₃PO₄, protonated amines such asN(H/R)_(3-n)[HX] where X comprises halides, nitrates, phosphates, orsulfates; n=0, 1, or 2; and R comprises an alkyl hydrocarbon chain, andcombinations of the foregoing.

In one embodiment, a mixture of silane precursors including atetraalkoxysilane, an organosilane, and an organofluorosilane may beused with isopropanol as a solvent in water and with 0.04 M HCl (acid)or 0.05 M NH₄OH (base) as a catalyst. In one embodiment, a mixture ofsilane precursors including tetramethoxysilane or tetraethoxysilane,methyltrimethoxysilane, and trifluoropropyltrimethoxysilane may be usedwith isopropanol as a solvent in water and with 0.04 M HCl (acid) or0.05 M NH₄OH (base) as a catalyst. In some embodiments that containprehydrolyzed organosilanes as the silane precursor, an acid catalystmay be used. In this case, the sol is a viscous liquid that can be usedfor making or applying the coating by rolling, screen printing, or byuse of a brush or other mechanical implements to spread the sol evenlyon the surface of a substrate.

The coating mixture may also optionally include other components toimprove performance of the final coating or to improve stability andshelf life of the sol mixture prior to its application on a substrate.These additional components may include low molecular weight polymers,refractive index tuning additives, cross-linking additives, and surfacemodifying additives. It should be appreciated that these optionaladditives may be used separately or in any combination in a coatingmixture. Further, it should be appreciated that these additionalcomponents can be used in any combination with any of the silaneprecursors and with any combination of solvents and acids or basesdescribed above.

In some embodiments, the coating mixture from which the sol is made mayoptionally include a low molecular weight polymer. The polymer functionsas a binder for mechanical stability of the coatings. It also helps withthe uniform spreading of the sol liquid on a substrate to provide ahomogeneous coating. In some embodiments, depending upon the intendeduse of the final coating, the amount of the polymer can vary from 0.1%to about 10%, from about 0.1% to about 1%, and from about 0.2% to about0.5%, by weight of the of the coating mixture from which the sol ismade. Examples of polymers that may be used includepoly(3,3,3-trifluoropropylmethylsiloxane),tridecafluorooctylmethylsiloxane, and dimethylsiloxane copolymer, aswell as combinations of these polymers. It also should be appreciatedthat combinations of polymers and acid catalysts may be used.

In some embodiments, the coating mixture from which the sol is made mayoptionally include a particle size modifying additive. As the refractiveindex (“RI”) of the coating affects the light transmission through thecoating or the anti-reflective behavior of the coating, components canbe added to the coating mixture to tune the particle size distributionin the sol to adjust the refractive index of the coating to increaselight transmission through the coating, thereby enhancing theanti-reflective ability of the coating. Non-limiting examples of theparticle size modifying additives include sodiumacetate-tris(hydroxymethyl)aminomethane or “TRIS” (—N-(2-acetamido)iminodiacetic acid-(4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid)or “HEPES”-, 2-(N-morpholino)ethanesulfonic acid or“MES”-imidazole-propanolamine ethylenediamine-diethylenetriamine- and3-aminopropyltrimethoxysilane-. In some embodiments, the use of theparticle size tuning additives increases the light transmittance fromabout 0.5% to about 2.0% compared to a coating without a particle sizetuning additive. The amount of particle size tuning additive used mayrange from about 0.01% to about 10% by weight of the coating mixture. Insome embodiments, the range is from about 0.1% to about 2% by weight ofthe coating mixture. Example 2 below describes the use of a particlesize modifying additive.

It should be appreciated that the anti-reflective property of the finalcoating is determined by both the coating composition and the coatingthickness. A particularly surprising and advantageous feature of using arefractive index tuning additive is that in addition to enhancing theanti-reflective properties of the coatings, it also enhances thespreadability of the sol, which leads to the formation of better qualityfilms. The hydrogen bonding groups on these refractive index tuningadditives are effective at stabilizing sol particles via prevention ofinter-particle aggregation. Because of their hydrogen bonding propertiesthey alter the viscosity of the sol and make it suitable to form filmswith the desired thickness, which is in the range of 100-150 nm in someembodiments. Because of their polar and hydrophilic nature they alsoalter the interactions of the sol with glass substrates and, therefore,lower the surface tension of the sol to enable better spreading. Therefractive index tuning additives also hydrogen bond with solventmolecules and, therefore, slow the rate of evaporation to enable uniformand homogeneous formation of coatings.

In some embodiments, the coating mixture from which the sol is made mayoptionally include a cross-linking additive to improve the abrasionresistance of the final coating. The abrasion resistance of theanti-reflecting coating is necessary for overall stability of thecoating and for long term performance. The abrasion resistance of acoating relates to its ability to withstand the action of externalmechanical forces that can lead to nicks and scratches, indentations,peeling, and associated loss of material from the coating. At a bulklevel the abrasion resistance relates to hardness and/or stiffness ofthe coating, which in turn relates to the ability of the sol to form anextended network via cross-linking. The abrasion resistance of the finalcoating, therefore, can be enhanced by means of cross-linking additivesthat can increase the interactions between the particles by formingcovalent linkages.

These optional cross-linking additives include silane coupling agents ofthe type α,ω-bis(trialkoxysilane). Specific examples of cross-linkingagents from this category include bis(trimethoxysilyl)methane,bis(trimethoxysilyl)ethane, bis(trimethoxysilyl)hexane,bis(trimethoxysilyl)octane, bis(trimethoxysilylethyl)benzene,bis[(3-methyldimethoxysilyl)propyl]propylene oxide. Optionally, thecross-linking agent can be of the trifunctional type such astris(3-trimethoxysilylpropyl)isocyanurate. Without being bound bytheory, it is believed that these additives when added to the coatingmixture increase cross-linking between particles and enhance themechanical properties of the coatings.

These cross-linking additives can be added to the coating mixture fromwhich the sol is formed or, in the case where separate sols are combinedto form a final sol, the cross-linking additives can be added to thatfinal sol. The amount of cross-linking additive used depends upon theamount of silane precursors used and is relative to the total amount ofall the silane precursors used. The cross-linking additive to totalsilane ratio can vary from about 0.1 to about 1. In some embodiments,the cross-linking additive to total silane ratio is from about 0.2 toabout 0.8. For trifunctional silane cross-linking additives, thecross-linking additive to total silane ratio is on the lower side ofthese ranges, while for bi-functional cross-linking additives, thecross-linking additive to total silane ratio it is on the higher side.Example 3 below describes the use of a cross-linking additive.

The coating composition may also optionally comprise surface modifyingadditives. Typically, the surface modifying additives would be usedinstead of a cross-linking additive. The surface modifying additivescomprise a two component system with complementary functional groupsthat can react with each other to form a covalent bond. The surfacemodification of the coating is carried out by a two-step method. First,an active precursor with cross-linkable functional groups (e.g., asilane coupling agent with reactive functional groups) is incorporatedinto the sol mixture to form a coating containing the functional groups.Second, the coating is treated with a hydrophobic reactive agent thatcan react with the embedded functional groups. This second step isperformed after the sol has gelled and as any solvent evaporates. Insome embodiments, this may take from about 30 seconds to about 2minutes. Once the gel sets, the gel is immersed in the hydrophobicreactive agent to initiate a reaction between the active precursor withcross-linkable functional groups (e.g., a silane coupling agent withreactive functional groups) and the hydrophobic reactive agent.Therefore, the active precursor with cross-linkable functional groups isincorporated into the coating followed by treatment of the coating withthe hydrophobic reactive agent that can react with the functional groupsin the coating to form a surface layer with hydrophobic groups on thesurface.

The active precursor with cross-linkable functional groups may be asilane coupling agent incorporated into the sol that has functionalgroups such as alcohol or silanol (—OH) aldehyde (—CHO), or isocyanate(—NCO) while the hydrophobic reactive agent has amine (—NH or —NH₂groups), silanol (—OH), or alkoxysilane groups. The silane couplingagent may be alkoxysilane precursors with functional groups that canreact with the hydrophobic reactive agent. Nonlimiting examples of thesilane coupling agent include triethoxysilyl butyraldehyde,hydroxymethyltriethoxysilane, 3-isocyanatopropyltriethoxysilane,3-isocyanatopropyltrimethoxysilane, isocyanatomethylldimethoxysilane,and isocyanatomethylldiethoxysilane. Examples of the hydrophobicreactive agent include hexamethyldisilazane, hexamethylcyclotrisilazane,and dichlorohexamethyltrisiloxane. The amount of active precursor withcross-linkable functional groups used depends upon the total silaneprecursor present in the coating mixture. The active precursor to totalsilane precursor ratio can vary from about 0.1 to about 1. In someembodiments, the active precursor to total silane precursor ratio variesfrom about 0.2 to about 0.8.

Since the hydrophobic reactive agent is applied to the gel state of thecoating mixture, the reactions that occur take place nominally on thesurface of the gel. Therefore, the hydrophobic reactive agent is notapplied in stoichiometric proportions. Typically, an about 1% to about5% by weight solution of IPA is applied to the gel surface. In someembodiments, an about 1% to about 2% by weight solution of IPA isapplied to the gel surface.

Example 4 below describes the use of a surface modifying additive. Itshould be appreciated that in some embodiments, the surface modificationof the coating retains the optical and mechanical properties of thecoating but increases surface contact angle by up to 15 degrees, whichimproves the anti-soiling or self-cleaning properties of the coating.

Table 1 lists various coating mixtures according to various embodimentsof the present disclosure.

TABLE 1 Fluorosilane Organosilane Solvent¹ Acid Polymer² (vol. %) (vol.%) (vol. %) (vol. %) (vol. %)(tridecafluoro-1,1,2,2-tetrahydrooctyl)tri- ethoxysilane (100%)(tridecafluoro-1,1,2,2-tetrahydrooctyl)di- methylchlorosilane (100%)(3,3,3-trifluoropropyl)tri-methoxysilane (100%)(3,3,3-trifluoropropyl)tri-chlorosilane (100%)(3,3,3-trifluoropropyl)methyldi- methoxysilane (100%)methyltrichlorosilane (100%) methyltrimethoxysilane (100%)phenylmethyldi-ethoxysilane (100%)(tridecafluoro-1,1,2,2-tetrahydrooctyl)tri- I (50%) ethoxysilane (50%)(tridecafluoro-1,1,2,2-tetrahydrooctyl)di- I (50%) methylchlorosilane(50%) (3,3,3-trifluoropropyl)tri-methoxysilane I (50%) (50%)(3,3,3-trifluoropropyl)methyldi- I (50%) methoxysilane (50%)(3,3,3-trifluoropropyl)tri-chlorosilane (50%) I (50%)methyltrichlorosilane (50%) I (50%) methyltrimethoxysilane (50%) I (50%)methyltrimethoxysilane (33%) I (33%)(tridecafluoro-1,1,2,2-tetrahydrooctyl)tri- methyltrimethoxysilane (33%)I (33%) ethoxysilane (33%) (tridecafluoro-1,1,2,2-tetrahydrooctyl)tri-dimethyldimethoxysilane I (33%) ethoxysilane (33%) (33%)(tridecafluoro-1,1,2,2-tetrahydrooctyl)tri- n-propylmethyldichloro- I(33%) ethoxysilane (33%) silane (33%) (3,3,3-trifluoropropyl)methyldi-methyltrimethoxysilane (33%) I (33%) methoxysilane (33%)(3,3,3-trifluoropropyl)tri-methoxysilane methyltrimethoxysilane (33%) I(33%) (100%) (tridecafluoro-1,1,2,2-tetrahydrooctyl)tri-methyltrimethoxysilane (33%) I (32%) 0.04 m HCl ethoxysilane (33%) (1%)(3,3,3-trifluoropropyl)tri-methoxysilane methyltrimethoxysilane (33%) I(32%) 0.04 m HCl (33%) (1%) (tridecafluoro-1,1,2,2-tetrahydrooctyl)tri-dimethyldimethoxysilane I (32%) 0.04 m HCl ethoxysilane (33%) (33%) (1%)(tridecafluoro-1,1,2,2-tetrahydrooctyl)tri- methyltrimethoxysilane (33%)I (32%) A (1%) ethoxysilane (33%)(tridecafluoro-1,1,2,2-tetrahydrooctyl)tri- dimethyldimethoxysilane I(32%) A (1%) ethoxysilane (33%) (33%)(3,3,3-trifluoropropyl)tri-methoxysilane methyltrimethoxysilane (33%) I(32%) A (1%) (33%) (tridecafluoro-1,1,2,2-tetrahydrooctyl)tri-dimethyldimethoxysilane I (32%) B (1%) ethoxysilane (33%) (33%)(tridecafluoro-1,1,2,2-tetrahydrooctyl)tri- methyltrimethoxysilane (33%)I (32%) B (1%) ethoxysilane (33%)(3,3,3-trifluoropropyl)tri-methoxysilane methyltrimethoxysilane (33%) I(32%) B (1%) (33%)

Other specific coating compositions according to various embodiments ofthe present disclosure include the following: (1) 0.38%tetramethoxysilane, 0.47% methyltrimethoxysilane, 0.56% trifluoropropyltrimethoxysilane, 0.05% HCl, 12.17% water, and 86.37% isopropanol; (2)0.38% tetramethoxysilane, 0.47% methyl trimethoxysilane, 0.56%trifluoropropyl trimethoxysilane, 0.04% NH₄OH, 12.18% water, and 86.37%isopropanol; and (3) 0.34% tetramethoxysilane, 0.47% methyltrimethoxysilane, 0.56% trifluoropropyl trimethoxysilane, 0.05% HCl,12.18% water, and 86.40% isopropanol, where all percentages are weightpercent.

The methods for preparing or formulating the coating compositions ormixtures from which the final sol is prepared will now be described. Asnoted above, the composition of the coating composition is based on useof a silane precursor or mixture of silane precursors. In those cases inwhich only one silane precursor is used, the process for preparing thesol basically includes mixing the silane precursor and any solventfollowed by the addition of any catalyst and then any other components.In those cases in which multiple silane precursors are used, the silaneprecursors can be mixed and hydrolyzed together or they can behydrolyzed separately and then mixed together prior to coating asubstrate. The strategy of hydrolyzing the precursors separately makesit possible to control the amounts of each precursor and also preventsextensive aggregation between the silanol species to maintain the lowviscosity necessary for depositing thin films. It should be appreciatedthat it is also possible to hydrolyze some of the silane precursorstogether and hydrolyze one or more other silane precursors separatelyand then mix all of them together prior to coating a substrate.

For example, the compositions listed in Table 1 above are based on aone-pot reaction or batch process. The components are mixed together toform the liquid or sol that is deposited onto the substrate. The orderof mixing includes mixing the solvent and silane precursor(s), thenadding acid (if any) and then polymer (if any). In other embodiments inwhich only one silane precursor is used, the method involves adding thesilane precursor, then the acid or base catalyst, and then water,followed by sonication. For other coating compositions, including thosedescribed in Examples 1, 2, and 3 below and those that use multiplesilane precursors, the silanes can be hydrolyzed separately and thenmixed together, or alternatively, they can be hydrolyzed together as aone-pot reaction system or batch process. In the latter case, the silaneprecursor(s) are added to the solvent followed by the addition of water.Then any acid or base catalyst is added followed by sonication. Itshould be appreciated that either a one-pot reaction to form the sol orformation of separate sols can be used; however, if multiple silaneprecursors are used, separate sol formation following by mixing providesbetter control over formation of the final mixed sol.

In one embodiment in which three silane precursors are used, a typicalmethod of preparation involves hydrolysis of two of the silaneprecursors, such as an organosilane and an organofluorosilane,separately in isopropanol under acidic (or basic) conditions. In thiscase, each of the two silane precursors is separately mixed with asolvent and any acid or base in the amounts described above. Dependingupon the total quantity being processed, mixing can be done by anymethod known in the art, including shaking, swirling, or agitation underambient conditions. For larger volumes a larger scale mixer may be used.

After mixing, each of the two individual sols is prepared by hydrolyzingthe respective silane precursor in an ultrasonic bath maintained atambient conditions for 30 minutes. In other words, the first mixture oforganosilane, solvent, and acid/base is placed in an ultrasonic bath,and the second mixture of organofluorosilane, solvent, and acid/base isseparately placed in an ultrasonic bath. Sonication is used to enablemixing at a molecular level for the hydrolysis reaction to take placebetween water and the silane precursor, which are immiscible. Avariation of the process involves hydrolysis in an ultrasonic bath at anelevated temperature to accelerate the hydrolysis process. In someembodiments, the temperature may be in the range of from about 40° C. toabout 80° C. When hydrolyzed, these silane precursors form a particulatesuspension of particles in a liquid or sol.

After being hydrolyzed, the two sols (e.g., one sol containinghydrolyzed organosilane and one sol containing hydrolyzedorganofluorosilane) are mixed together along with a third silaneprecursor, for example, a tetraalkoxysilane, and the entire mixture isfurther sonicated for 30 minutes in the ultrasonic bath. The combined orfinal sol is then allowed to age under ambient conditions for a variableamount of time from 30 minutes to 48 hours. Aging is required for thesol-gel reactions (hydrolysis and condensation) to go to completion andfor the sol to obtain a steady state condition. Aging time alsofacilitates reaction between particles from the different sols to form ahomogeneous mixture. The relative extent of aging dictates sol viscosityand consequently film homogeneity and film thickness, which in turn,controls the anti-reflective properties of the coating. In someembodiments, longer aging times of 24-48 hours may be used since theyallow sufficient time for the system to reach equilibrium. Shorter agingtimes may be used to prevent formation of bigger particles to obtain atargeted viscosity and coating thickness upon application of the sol toa substrate for sol systems that are too reactive or too concentrated.

Variations of this method may be made in some embodiments. For example,the final sol can be made by mixing all of the silane precursorsconcurrently in one container and hydrolyzing them together in onereaction. Alternatively, when using three silane precursors (e.g., anorganosilane, an organofluorosilane, and a tetraalkylsilane) threeseparate sols can be formed and then mixed together to form the finalsol, as opposed to forming two sols and then mixing these with a thirdsilane precursor.

Also, the sonication times can vary from 30 minutes to 5 hours and canbe done at room temperature conditions or can be done at elevatedtemperatures of up to 80° C. to facilitate to completion the sol-gelreactions of hydrolysis and condensation. Longer sonication times arepreferred since they ensure that the reactions have reached steady-stateequilibrium. Increasing temperature also helps the reactions to go tocompletion, which permit the use of shorter sonication times. Increasedtemperature may also assist in forming smaller particles, since thereaction rates may be significantly enhanced at higher temperature toform a large number of smaller particles as opposed to a smaller numberof large particles. In some embodiments, small particles in the range of1-10 nm are preferred for forming coatings with optimum anti-reflectionproperties. Also, as noted the aging time for the sol can vary from 30minutes to 48 hours to allow sufficient time for the system to reachequilibrium and to produce the desired particle size and viscosity. Inother words, the sonication time and temperature and the aging time canbe adjusted to produce an equilibrated sol having the desired particlesize, which in some embodiments is particles having a size of about 1 nmto about 10 nm, and viscosity. The application of the coating mixturesabove will now be described. The coating mixtures described herein areused to form a uniform, homogenous, optical quality, crack-free coatingthat is largely devoid of defects and imperfections. The methods offabricating a durable coating are based on a combination of factorsincluding the composition of the coating mixture, the use of appropriatesolvents or combination of solvents, substrate preparation, coatingmethods, and specific curing conditions.

In some embodiments, for example, the coating mixtures described inTable 1, the coating mixtures of the present disclosure may be in theform of a stable liquid composition that can be applied to a givensubstrate. The coating mixture that is applied to a substrate may alsobe in the form of a gel, lotion, paste, spray, or foam. In someembodiments, the coating mixture that is applied to a substrate is inthe form of a liquid or viscous fluid or clear liquid. In someembodiments, the coating compositions are present as a clear liquid foruse as a spray, or alternatively, as a dispersion, viscous liquid, or amixture of these. In some embodiments, the coating mixtures have aviscosity in the range of approximately 0.5-5 cP for clear liquidcompositions and approximately 10-200 cP in the form of pre-hydrolyzedviscous liquid, where pre-hydrolyzed refers to a liquid sol containinghydrolyzed particles or hydrolyzed silane precursors. It should beappreciated that in some cases, the coating mixture can be applied tothe substrate before hydrolysis. In other words, the coating mixture isapplied to the substrate after which the coating mixture is hydrolyzedand condensed with the aid of moisture in the air. Accordingly, the solis formed on the substrate followed by formation of the final coating.It should be appreciated that the state of the coating mixture forapplication on a substrate can be determined based upon the form mostapplicable to form a thin film or coating in conjunction with themechanical or physical method used for actually applying the coatingmixture.

It should be appreciated that the coating material and process by whichit is applied to the substrate comprise a larger coating system. Thecoating material is optimized for a particular coating method and viceversa. Thus the optimized coating process is preferentially performed bya custom designed tool to insure consistency and quality. Therefore thistool coupled with the coating materials comprises the coating system.Given that the benefits of the current disclosure are particularly wellsuited to solar panel manufacturers, who do not themselves manufacturetools, it is desirable to offer a complete solution consisting of boththe coating material and its associated coating tool. In the followingparagraphs describing the coating process it should be appreciated thatthese steps could be executed manually, automatically using a coatingtool or in any combination of both.

Furthermore, the custom designed coating tool may be a large stand-aloneunit intended for operation in a factory setting; it could be asub-tool, such that it comprises a process module that performs thecoating process but that is integrated into another machine thatperforms other steps in the larger solar panel manufacturing process.For example it could be a module attached to an existing glass washingmachine or a module attached to a panel assembly machine. Alternatively,the tool could be portable or semi-portable for example mounted on atruck or inside a tractor trailer such that it could be transported to aworksite and used to coat solar modules during the construction of alarge solar installation. Alternatively it could be designed such thatthe coating could be applied to installed solar modules in situ.

In general, three steps are used to apply the sol to a given substrate.First, the substrate is cleaned and prepared. Second, the substrate iscoated with the sol or mixture of sols. Third, the final coating isformed on the substrate.

As an initial step, the substrate is pre-treated or pre-cleaned toremove surface impurities and to activate the surface by generating afresh surface or new binding sites on the surface. The substratepre-treatment steps are important in providing uniform spreading anddeposition of the sol, effective bonding interactions between thesubstrate and coating material for Si—O—Si linkage formation, andprevention of defects and imperfections at the coating-substrateinterface because of uneven spreading and/or diminished bondinginteractions due to surface inhomogeneities.

In particular, it is desirable to expose Si—OH groups on the surface ofthe substrate through pre-treatment or cleaning of the substrate surfaceto form an “activated” surface. An activated surface layer lowers thesurface tension of the predominantly hydrophilic solvents in the sol andenables effective spreading of the sol on the surface. In someembodiments, a combination of physical polishing or cleaning and/orchemical etching is sufficient to provide even spreading of the sol. Incases, where the surface tension would need to be further lowered, thesubstrate, such as glass, may be pretreated with a dilute surfactantsolution (low molecular weight surfactants such as surfynol; long chainalcohols such as hexanol or octanol; low molecular weight ethylene oxideor propylene oxide; or a commercial dishwasher detergent such asCASCADE, FINISH, or ELECTRASOL to further help the sol spread better onthe glass surface.

Accordingly, surface preparation involves a combination of chemical andphysical treatment of the surface. The chemical treatment steps include(1) cleaning the surface with a solvent or combination of solvents, (2)cleaning the surface with a solvent along with an abrasive pad, (3)optionally chemically etching the surface, and (4) washing the surfacewith water. The physical treatment steps include (1) cleaning thesurface with a solvent or combination of solvents, (2) cleaning thesurface with a solvent along with particulate abrasives, and (3) washingthe surface with water. It should be appreciated that a substrate can bepre-treated by using only the chemical treatment steps or only thephysical treatment steps. Alternatively, both chemical and physicaltreatment steps could be used in any combination. It should be furtherappreciated that the physical cleaning action of friction between acleaning brush or pad and the surface is an important aspect of thesurface preparation.

In the first chemical treatment step, the surface is treated with asolvent or combination of solvents with variable hydrophobicity. Typicalsolvents used are water, ethanol, isopropanol, acetone, and methyl ethylketone. A commercial glass cleaner (e.g., WINDEX) can also be employedfor this purposes. The surface may be treated with an individual solventseparately or by using a mixture of solvents. In the second step, anabrasive pad (e.g., SCOTCHBRITE) is rubbed over the surface with the useof a solvent, noting that this may be performed in conjunction with thefirst step or separately after the first step. In the last step, thesurface is washed or rinsed with water.

One example of substrate preparation by this method involves cleaningthe surface with an organic solvent such as ethanol, isopropanol, oracetone to remove organic surface impurities, dirt, dust, and/or grease(with or without an abrasive pad) followed by cleaning the surface withwater. Another example involves cleaning the surface with methyl ethylketone (with or without an abrasive pad) followed by washing the surfacewith water. Another example is based on using a 1:1 mixture of ethanoland acetone to remove organic impurities followed by washing the surfacewith water.

In some instances an additional, optional step of chemically etching thesurface by means of concentrated nitric acid, sulfuric acid, or piranhasolution (1:1 mixture of 96% sulfuric acid and 30% H₂O₂) may benecessary to make the surface suitable for bonding to the deposited sol.Typically this step would be performed prior the last step of rinsingthe surface with water. In one embodiment, the substrate may be placedin piranha solution for 20 minutes followed by soaking in deionizedwater for 5 minutes. The substrate may then be transferred to anothercontainer holding fresh deionized water and soaked for another 5minutes. Finally, the substrate is rinsed with deionized water andair-dried.

The substrate may alternatively or additionally prepared by physicaltreatment. In the physical treatment case, for one embodiment thesurface is simply cleaned with a solvent and the mechanical action of acleaning brush or pad, optionally a surfactant or detergent can be addedto the solvent, after which the substrate is rinsed with water and airdried. In another embodiment the surface is first cleaned with waterfollowed by addition of powdered abrasive particles such as ceria,titania, zirconia, alumina, aluminum silicate, silica, magnesiumhydroxide, aluminum hydroxide particles, or combinations thereof ontothe surface of the substrate to form a thick slurry or paste on thesurface. The abrasive media can be in the form a powder or it can be inthe form of slurry, dispersion, suspension, emulsion, or paste. Theparticle size of the abrasives can vary from 0.1 to 10 microns and insome embodiments from 1 to 5 microns. The substrate may be polished withthe abrasive slurry via rubbing with a pad (e.g., a SCOTCHBRITE pad), acloth, or paper pad. Alternatively, the substrate may be polished byplacement on the rotating disc of a polisher followed by application ofabrasive slurry on the surface and rubbing with a pad as the substraterotates on the disc. Another alternative method involves use of anelectronic polisher that can be used as a rubbing pad in combinationwith abrasive slurry to polish the surface. The substrates polished withthe slurry are cleaned by pressurized water jet and air-dried.

After pretreating the surface, the final sol is deposited on a substrateby techniques known in the art, including roll coating, dip coating,spraying, drop rolling, or flow coating to form a uniform coating on thesubstrate. Other methods for deposition that can be used includespin-coating; aerosol deposition; ultrasound, heat, or electricaldeposition means; micro-deposition techniques such as ink-jet, spay-jet,xerography; or commercial printing techniques such as silk printing, dotmatrix printing, etc. Deposition of the sol is typically done underambient conditions.

In some embodiments, the method of deposition is performed via the droprolling method on small surfaces wherein the sol composition is placedonto the surface of a substrate followed by tilting the substrate toenable the liquid to roll across the entire surface. For largersurfaces, the sol may be deposited by flow coating wherein the sol isdispensed from a single nozzle onto a moving substrate at a rate suchthat the flowing sol leads to a uniform deposition onto a surface orfrom multiple nozzles onto a stationary surface or from a slot onto astationary surface. Another method of deposition is via depositing theliquid sol onto a substrate followed by use of a mechanical dispersantto spread the liquid evenly onto a substrate. For example, a squeegee orother mechanical device having a sharp, well-defined, uniform edge maybe used to spread the sol.

In addition to the actual methods or techniques used to deposit thefinal sol on the substrate, it should be appreciated that severalvariations for depositing the final sol exist. For example, in oneembodiment, the final sol is simply deposited on the substrate in onelayer. In another embodiment, a single sol or multiple sols may bedeposited to form multiple layers, thereby ultimately forming amultilayered coating. For example, a coating of the sol containing anorganosilane and an organofluorosilane can be formed as an underlayerfollowed by a topcoat of a tetraalkoxysilane. In another embodiment, anunderlayer of an organosilane may be deposited followed by thedeposition of a topcoat of a mixture of an organofluorosilane and atetraalkoxysilane. In another embodiment, an underlayer of atetraalkoxysilane may be deposited followed by the deposition of a toplayer using a sol mixture of an organosilane and an organofluorosilane.In another embodiment, an underlayer of a sol made from a mixture of anorganosilane and an organofluorosilane may be deposited followed byvapor deposition of a top layer by exposing the layer to vapors of atetraalkoxysilane. In another embodiment, an underlayer of a sol madefrom a mixture of an organosilane and an organofluorosilane may bedeposited followed by deposition of a top layer by immersing thesubstrate in a solution of a tetraalkoxysilane in isopropanol. In theembodiments in which multiple layers are deposited, each layer may bedeposited almost immediately after deposition of the first layer, forexample, within or after 30 seconds of deposition of the prior layer. Asnoted, different sols may be deposited on top of one another, ordifferent mixtures of sols may be deposited on top of one another.Alternatively, a single sol may be deposited in multiple layers or thesame sol mixture may be deposited in multiple layers. Further, a givensol may be deposited as one layer and a different sol mixture may beused as another layers. Further, it should be appreciated that anycombination of sols may be deposited in any order, thereby constructinga variety of multi-layered coatings. Further, it should be appreciatedthat the sols for each layer may be deposited using different techniquesif so desired.

The thickness of the coatings deposited can vary from about 10 nm toabout 5 micron. In some embodiments, the thickness of the coating variesfrom about 100 nm to about 1 micron, and in other embodiments it variesfrom about 100 nm to about 500 nm. In order to provide sufficientanti-reflective properties, a thickness of about 60 nm to about 150 nmis desired. It should be appreciated that the thickness of the coatingmixture as deposited is affected by the coating method, as well as theviscosity of the coating mixture. Accordingly, the coating method shouldbe selected so that the desired coating thickness is achieved for anygiven coating mixture. Further, in those embodiments, in which multiplelayers of sols are deposited, each layer should be deposited in athickness such that the total thickness of the coating is appropriate.Accordingly, in some embodiments in which multiple layers of sols aredeposited, the overall coating thickness varies from about 100 nm toabout 500 nm, and in order to provide sufficient anti-reflectiveproperties, a total coating thickness of about 60 nm to about 150 nm isdesired.

Once the final sol is deposited as described above, the deposited solwill proceed to form a gel through the process of gelation after whichthe gel is dried and cured to remove residual solvent and facilitatenetwork formation via Si—O—Si linkage formation in the coating. Inaddition, the gel may be allowed to age to allow for the formation ofadditional linkages through continued hydrolysis and condensationreactions.

As described above, the sol-gel method used in preparing the coatingsdescribed herein utilizes a suitable molecular precursor that ishydrolyzed to generate a solid-state polymeric oxide network. Initialhydrolysis of the precursor generates a liquid sol, which ultimatelyturns to a solid, porous gel. Drying of the gels under ambientconditions (or at elevated temperature) leads to evaporation of thesolvent phase to form a cross-linked film. Accordingly, throughout theprocess, the coating mixture/sol/gel/dried/cured coating undergoeschanges in physical, chemical, and structural parameters thatintrinsically alter the material properties of the final coating. Ingeneral, the changes throughout the sol-gel transformation can beloosely divided into three interdependent aspects of physical, chemical,and structural changes that result in altered structural composition,morphology, and microstructure. The chemical composition, physicalstate, and overall molecular structure of the sol and the gel aresignificantly different such that the materials in the two states areintrinsically distinct.

Regarding physical differences, the sol is a collection of dispersedparticles suspended in a liquid. These particles are surrounded by asolvent shell and do not interact with each other significantly. Assuch, the sol is characterized by fluidity and exists in a liquid state.In contrast, in a gel film the network formation has occurred to anadvanced state such the particles are interconnected to each other. Theincreased network formation and cross-linking makes the gel networkrigid with a characteristic solid state. The ability of the material toexist in two different states is because of the chemical changes thatoccur along the sol to gel transformation.

Regarding chemical changes, during the sol to gel transition, the solparticles combine with each other via formation of Si—O—Si linkages. Asa result, the material exhibits network formation and strengthening.Overall, the sol particles contain reactive hydroxyl groups on thesurfaces that can participate in network formation while the gelstructure has these hydroxyl groups converted into siloxane groups.

Regarding structural differences, the sol contains discrete particlescontaining few siloxane linkages along with terminal hydroxyl as well asunhydrolyzed alkoxy ligands. As such, the sol state can be consideredstructurally different from the solidified films which contain majoritysiloxanes. As such, the liquid sol and the solid state polymericnetworks are chemically and structurally distinct systems.

Regarding differences in properties, the origin of the physical andchemical properties of the sol and gel films depends upon theirstructure. The sol particles and the gel films differ in the chemicalcomposition, makeup and functional groups and as a result exhibitdifferent physical and chemical properties. The sol stage because of itsparticulate nature is characterized by high reactivity to form thenetwork while the gel state is largely unreactive due to conversion ofreactive hydroxyl groups to stable siloxane linkages. Accordingly, itshould be appreciated that it is the particular combination of silaneprecursors and other chemicals added to the coating mixture that ishydrolyzed and condensed, gelled, dried and cured on a substrate surfacethat gives the final coatings of the present disclosure the desiredproperties described above.

There are several methods by which the gel is dried and cured and/oraged to form the final coating. In some embodiments the gel is dried andcured under ambient or room temperature conditions. In some embodiments,the gel is aged under ambient conditions for 30 minutes followed bydrying for 3 hours in an oven kept at a variable relative humidity of(e.g., 20% to 50%). The temperature of the oven is then increased slowlyat a rate of 5° C./min to a final temperature of 120° C. The slowheating rate along with the moisture slows the rate of the silanolcondensation reaction to provide a more uniform and mechanically stablecoating. This method provides reproducible results and is a reliablemethod of making the coating with the desired properties.

In another embodiment, the gel on the substrate is heated under aninfrared lamp or array of lamps. These lamps are placed close proximityto the substrate's coated surface such that the surface is evenlyilluminated. The lamps are chosen for maximum emission in themid-infrared region of 3˜5 um wavelength. This region is desirablebecause it is adsorbed better by glass than shorter infraredwavelengths. The power output of the lamps may be closely controlled viaa closed loop PID controller to achieve a precise and controllabletemperature profile. In some embodiments this profile will start fromambient temperature and quickly rise 1˜50 degrees centigrade per secondto a temperature of 120 degrees centigrade, hold that temperature for aperiod of 30 to 300 seconds, then reduce temperature back to ambient,with or without the aid of cooling airflow.

In another embodiment, the coated substrate is heated on a hot plate at120° C.° C. such that the uncoated surface is in contact with the hotplate while the coated surface is exposed to air. In this case, the hotplate is turned on to a set temperature of 120° C. after the substratehas been placed on the hot plate. This ensures a slow heating rate toprevent cracking, flaking, and/or delamination of the coating material.

In another embodiment, the coating on the substrate is heated on a hotplate at 200-300° C. wherein the surface of the hot plate is coveredwith a silicone mat that is in direct contact with the surface of thecoating to reach a desired curing temperature of 120° C. within 10-30seconds. The conductive nature of the silicone mat makes it conducive totransfer heat to the coated surface to increase the continuinghydrolysis and condensation reactions. In this case, the coated surfacecan reach a temperature of 120° C. within 10-30 seconds.

It is particularly noteworthy that the coatings of this disclosure areprepared under temperatures not exceeding 120° C. in contrast totemperatures of 400-600° C. typically employed in curing silica-basedanti-reflective coatings. Another particularly advantageous feature ofsome of the coating compositions herein, particularly those of Table 1,as opposed to the coating mixtures that utilize more than one or morethan two silane precursors, is that they do not require water as aspecific component of the composition for the reaction or curing processto proceed. It is particularly advantageous that the coatingcompositions can be made to harden by reaction with moisture within theenvironment or alternatively by the trace amounts of water present inthe solvent. The curing of the coating in a humid environment slows downthe evaporation of water leading to a coating with improvedcross-linking and better mechanical properties.

As described above and as illustrated further in the Examples, thecoatings made as described herein have several desirable properties. Thecoatings have anti-reflective properties that reduce the reflection ofphotons. The transmittance of a glass substrate coated with a coatingcomposition made according to the present disclosure can vary from about92% to about 98%, from about 93% to about 96%, and from about 95% toabout 98%.

The coatings also have anti-soiling properties, which are also importantin maintaining sufficient transmittance when used in conjunction with aglass substrate. Soiling is due to adherence of particulate matter onsurfaces exposed to environment. The deposition of the particles ontosurfaces depends upon the surface microstructure as well as chemicalcomposition. In general, rough surfaces can provide many sites forphysical binding of particulate matter. For solar panels, soiling canlead to reduction in power output due to reduced absorption of light oftypically about 5% and in some cases losses of 22% have been reported.The paper “The Effect of Soiling on Large Grid-Connected PhotovoltaicSystems in California and the Southwest Region of the United States”,Photovoltaic Energy Conversion, Conference Record of the 2006 IEEE 4thWorld Conference, May 2006, Vol 2, p 2391-2395, reports an average 5%loss. The paper “Soiling and other optical losses in solar-tracking PVplants in Navarra”, Prog. Photovolt: Res. Appl. 2011; 19:211-217,reports losses of 22%.

The chemical composition of the surfaces is reflected in the surfaceenergy as measured by contact angles. Low energy surfaces (characterizedby high water contact angles) are usually less susceptible to binding ascompared to high energy surfaces with low water contact angles.Therefore, anti-soiling properties can be determined indirectly bymeasuring the coating's contact angle. The coatings herein providecontact angles ranging from about 80 degrees to about 178 degrees, fromabout 110 degrees to about 155 degrees, and from about 125 degrees toabout 175 degrees. The coatings of this disclosure were alsocharacterized for anti-soiling behavior by subjecting them to repeatedcycles of dirt exposure followed by air cleaning (see, for example,FIGS. 9 and 10 and Table 3 below). The coatings of this disclosureminimize the photon flux losses due to soiling by about 50% relative touncoated samples.

The coatings of the present disclosure also provide desirable mechanicalproperties. Nano-indentation is a method of used to measure themechanical properties of nanoscale materials especially thin films andcoatings. In nano-indentation measurements a load is applied to theindenter tip that provides the necessary force for the tip to be pushedinto the sample thereby creating a nanoscale indent on the sample.Nano-indentation measurements involve continuous measurement of theload, contact stiffness, and depth of penetration of the tip withrespect to time to record a load-displacement profile. The loaddisplacement curves in combination with the contact displacement can beused to measure mechanical properties such as hardness and Young'sModulus (or modulus of elasticity). Noting that the typical hardness fora mixture of pure silica sol-gels coatings is observed to be around 1.05GPa, the coatings of the present disclosure exhibit much greaterhardness (see, for example, FIGS. 7a, 7b, 8a and 8b and Table 3 below).In addition, the pencil hardness of the coating of the presentdisclosure can vary from about 2H to about 9H, from about 4H to about7H, and from about 6H to about 9H.

The coatings of the present disclosure also provide desirable abrasionresistance. Abrasion resistance can be defined as the ability of amaterial to withstand erosion due to frictional forces to preserve andmaintain its original shape and appearance. Abrasion resistance relatesto the strength of the intrinsic framework structure as well as tosurface features. Materials that do not have sufficient strength due tolack of long range bonding interactions tend to abrade easily.Similarly, materials with uneven surfaces or coatings with surfaceinhomogeneities and asperities tend to wear due to frictional losses.Also, the leveling and smoothening of these asperities due to frictionleads to changes in optical transmission of the coating as the materialis abraded.

The coatings of the present disclosure pass the standard test formeasuring abrasion resistance of coatings on surfaces as definedaccording to European Standard EN-1096-2 (Glass in Building, CoatedGlass). The test involves the action of rubbing a felt pad on the coatedglass. The felt rubbing pad is subjected to a to-and-fro translationmotion with a stroke length of 120±5 mm at a speed of 54-66 strokes/mincombined with a continuous rotation of the pad of 6 rpm or of a rotationof between 10° to 30° at the end of each stroke. The back and forthmotion along with the rotation constitutes 1 cycle. The specificationsof the circular felt rubbing pad include a diameter of 14-15 mm,thickness of 10 mm and density of 0.52 g/cm³. The felt pad is attachedto a mechanical finger that is 15 mm to 20 mm is diameter and placedunder a load of 4 Newtons. The transmission at 550 and 900 nm ismeasured to evaluate abrasion resistance and the standard dictates achange in transmission of no more that ±0.05 with respect to a referencesample.

In general, the various coatings of the present disclosure provide ameans of making a transparent substrate or glass transmit more photonswithout altering its intrinsic structure and other properties, alongwith passivating the surface so that it becomes resistant to theadhesion of water, dirt, soil, and other exogenous matter. Accordingly,the coating mixtures and resulting gels and coatings as described hereinhave varied commercial applications.

Regarding the coating mixtures themselves, these may be sold as acoating mixture or commercial coating formulation for others to use. Forexample, the coating mixtures may be provided as a liquid composition,for example, for subsequent small scale treatment of glass in atreatment separate from their usage as windows in solar or architecturalsystems. In this case the coating mixture may be sold before the silaneprecursors are hydrolyzed. Alternatively, the coating mixtures may besold as sols or after the silane precursors have been hydrolyzed.

In addition, the coating mixtures may be deposited and allowed to gel ona particular substrate that is subsequently sold. In particular, thecoating compositions of the present disclosure can be coated onto anytransparent substrate that has hydrogen bond donor or hydrogen bondacceptor groups on the surface. For example, the coating can be appliedas a treatment for a given glass or other transparent substrate beforeor after it has been integrated into a device, such a solar cell,optical window or enclosure, for example, as part of a glass treatmentprocess. In other embodiments, the disclosure provides for the use ofthe coating compositions as an efficiency enhancement aid inarchitectural windows in building and houses by the provision ofanti-reflection benefits and/or by the provision of anti-soilingbenefits to augment the anti-reflection benefits. In other embodiments,the disclosure provides for the use of the coating compositions as anefficiency enhancement aid in treatment of transparent surfaces thatrequire regular cleaning to make them self-cleaning. For example, thecoatings can be used in conjunction with glass used in windows,windshields, screens, architecture, goggles, eyeglasses, etc.

In other embodiments, the disclosure provides for the use of the coatingcompositions as an efficiency enhancement aid in photovoltaic solarpanel assemblies (e.g., the outer cover of solar panels) by theprovision of anti-reflection benefits and/or by the provision ofanti-soiling benefits to augment the anti-reflection benefits. Thesedevices convert solar energy into electrical energy and rely uponefficient absorption of photons, and effects such as reflection,scattering, and loss of absorption due to adsorbed soil or dirtparticles can lead to reduced power output. As noted, the coatings ofthis disclosure when coated onto a glass surface reduces reflection ofphotons (the so-called anti-reflective property) and also reducesadsorption and binding of dirt, soil, and other particulate matter fromthe environment to boost the transmission of photons through the glassas well as to prevent reduction in photons associated with deposition ofparticulate matter onto the surface.

The coatings for solar panel applications provide unique challenges thatare not present with coatings typically utilized in other commonapplications. The use of anti-reflective coating in solar panelsnecessitates long term exposure of solar radiation that usually resultsin extensive degradation of polymeric materials under prolonged UVexposure due to photolytic breakdown of bonds in these materials. Thecoating compositions of the present disclosure utilize silane precursorsthat when hydrolyzed and dried and cured give rise to a network that issimilar to glass with Si—O—Si bonds that are stable to radiativebreakdown. An additional advantage of using silica based materials insolar applications is the intrinsic hardness of the material that makesthe coating resistant to scratches, indentations, and abrasion. Further,the coatings of the present disclosure provide for enhanced lighttransmittance across the entire solar region from about 400 nm to about1150 nm, which is desirable for solar applications.

Further, it should be appreciated that the sols resulting from thecoating compositions of this disclosure do not need to be applied to thesolar panels during manufacturing and may be applied after manufacturingto avoid any interference with the solar panel manufacturing process. Itis expected that the solar panel maker themselves may be able to use thecomposition of this disclosure to coat the modules at appropriate pointswithin their manufacturing process. In such instances, the provision ofa stable sol, that can be used according to the methods describedherein, provides a direct means for the applying the coating mixtureafter manufacture of the panels or even after final installation of thepanels. This may streamline the manufacturing process and enhance theeconomic value of existing panels, either existing inventory or panelsalready installed and in use, to which the coatings can be applied.

Coating mixtures that can be used specifically for coating solar panelsinclude (1) 0.38% tetramethoxysilane, 0.47% methyl trimethoxysilane,0.56% trifluoropropyl trimethoxysilane, 0.05% HCl, 12.17% water, and86.37% isopropanol; (2) 0.38% tetramethoxysilane, 0.47% methyltrimethoxysilane, 0.56% trifluoropropyl trimethoxysilane, 0.04% NH4OH,12.18% water, and 86.37% isopropanol; and (3) 0.34% tetramethoxysilane,0.47% methyl trimethoxysilane, 0.56% trifluoropropyl trimethoxysilane,0.05% HCl, 12.18% water, and 86.40% isopropanol, where all percentagesare weight percents.

In one embodiment, the process of coating the solar panels consists ofpreparing the panel surface, coating the surface with the final sol madein accordance with the present disclosure, drying the coating underambient conditions, and curing the dried panels at elevated temperature.The panel surface is prepared by polishing the panel with a cerium oxideslurry, followed by washing the panel with water, and drying it underambient temperature-pressure conditions for a period ranging from about10 hours to about 12 hours.

Once the panel surface is prepared, in one embodiment, the final sol ofthe present disclosure is deposited onto solar panels by means of a flowcoater. The sol is deposited onto the panels via gravitational free flowof the liquid sol from top to bottom. The solar panels are placed on themobile platform that moves at a rate that is optimal for the free flowof the sol without introducing break points in the liquid stream orintroducing turbulent flow. The rate of liquid flow and the rate ofmovement of platform carrying the solar panel are optimized fordeposition of uniform, crack-free coatings that are homogenous, free ofdeformities, and characterized by uniform thickness.

More specifically, in one embodiment, the panel is placed on a mobilestage that is connected to a computer and programmed to move at a speedranging, in some embodiments, from about 0.05 cm/s to about 300 cm/s, inother embodiments from about 0.1 cm/s to about 10 cm/s, and in otherembodiments from about 0.25 cm/s to about 0.5 cm/s. The sol is thendeposited onto the panel surface using a nozzle dispensing unit (that isconnected to a computer) such that the rate of flow of sol is, in someembodiments, from about 5 ml/min to about 50 ml/min, in otherembodiments from about 5 ml/min to about 25 ml/min, and in otherembodiments from about 10 ml/min to about 15 ml/min. It should beappreciated that the rate at which the sol is deposited is important forproper deposition of the coatings. Notably, the nozzle diameter of thesol can be adjusted to ensure appropriate flow rate, with diameters ofthe nozzle ranging from about 0.3 mm to about 0.9 mm.

A particularly advantageous aspect of using a sol is that it is in aliquid state but is also viscous enough to spread without breakdown ofthe stream. The uniformity of the coatings is further ensured byadjusting the flow rate and the rate of the movement of the platformcontaining the solar panels. For a given flow rate of the sol, if therate of the movement of the platform is too fast then it leads torupture of the sol stream causing uneven coatings. For a given flow rateof the sol, if the rate of the movement of the platform is too slow itresults in turbulent flow that deteriorates the uniformity of the films.Therefore, a specific optimum of sol flow rate and the platform movementare important to provide even, uniform, and homogenous coatings. The useof specific pH, solvent, and silane concentrations as outlined aboveprovide the ideal viscosities.

The coating process is also facilitated by the evaporation of thesolvent during the flow of the sol onto the panel, which also affectsthe development of uniform films or coatings on the panel surface. Thecoatings are formed when the free flowing sol dries on the surface andforms a solid on the glass surface. More specifically, the bottom edgeof the sol represents the wet line while drying occurs at the top edge.As the solvent evaporates, the sol becomes more viscous and finally setsat the top edge while the bottom edge is characterized by liquid edgespreading. The spreading liquid at the bottom edge enables the free flowof the sol while the setting sol at the top edge fixes the materials andprevents formation of lamellar structures. A balance of these factors isimportant for formation of uniform films.

It should be appreciated that the flow coating method does not allowseepage of the sol into the internal parts of the solar panel assemblyas the excess sol can be collected into a container at the bottom of theassembly and recycled. Similarly, it does not facilitate corrosionand/or leaching of the chemicals from the interior of the solar panelassembly. The flow coater method exposes only the glass side to the solwhile the other side of the panel assembly with electrical contacts andleads does not come into any contact with the liquid sol. As such, theflow coating process is particular beneficial to coating solar panelsduring either the assembly or the post-assembly stages.

The methods described here can be used to coat solar panels of variablesizes and in variable configurations. For example, typical panels havethe dimensions of 1 m×1.6 m, which can be coated either in portraitconfiguration or in landscape mode via appropriate placement the mobileplatform.

The flow coater can be used to coat the panels at the rate of about15-60 panels per hour. The rate of coating of individual panels woulddepend upon the size of the panels and whether they are coated in theportrait mode or landscape orientation. Additionally, multiple coatersoperating in parallel can be used in conjunction with the panel assemblyline to increase the production rate.

After depositing the coating, the panel is dried for a period rangingfrom about 1 minutes to about 20 minutes or longer under ambientconditions. The coated panel is then cured using any of the techniquesfor curing described above, after which the coated panel is ready foruse.

The anti-reflecting coatings described herein increase the peak power ofthe solar cells by approximately 3% due to the anti-reflective property.In addition, it is estimated that the anti-soiling property wouldcontribute to minimize transmissive losses associated with accumulationof dirt on the panels. Typical soiling losses are estimated at about 5%and use of these coatings is expected to reduce the losses in half.

Examples

The following describes various aspects of the coatings made accordingto certain embodiments of the disclosure in connection with the Figures.These examples should not be viewed as limiting.

In one embodiment referred to as Example 1, Sol I was prepared by firstmixing 22.5 mL of isopropanol (IPA) and 2.5 mL of 0.04M HCl (pH 1.5).100 μL of methyltrimethoxysilane (MTMOS) was then added to this mixture.The final solution of IPA, HCl, and MTMOS was then sonicated in asonicator for 35 minutes. Sol II was prepared by first mixing 22.5 mL ofIPA and 2.5 mL of 0.04M HCl (pH 1.5) followed by adding 100 μL of(3,3,3-trifluoropropyl)-trimethoxysilane (F3TMOS). Sol II was alsosonicated for 35 minutes. After sonication, Sol I and Sol II was mixedin equal parts (12.5 mL each), and 100 μL of tetramethoxysilane (TMOS)was added. This final solution was then sonicated for 35 minutes. Thismixture was allowed to age under ambient conditions for 24 hours up to120 hours. After aging, microscope slides (polished with cerium oxidepolish, washed, and allowed to dry) were flow coated with the final solmixture and allowed to dry for approximately 5-10 minutes. Once dry, theslides were cured in one of two ways. In one method, the slides wereplaced coated side up on a hot plate/stirrer for 45 minutes at 120° C.In the other method, they were placed in an oven at 25° C. for 25minutes. The temperature was then raised at a constant rate over aperiod of 20 minutes to 120° C., and then maintained at 120° C. for 180minutes. The temperature was then cooled to 25° C. at a constant rateover a period of 60 minutes.

In one embodiment demonstrating the use of a particle size modifyingadditive (referred to as Example 2, a sol was prepared usingtris(hydroxymethyl)aminomethane (TRIS) as follows: A sol was preparedaccording to the process in Example 1. After final sonication, themixture was allowed to chill in refrigerated water for 5 minutes andthen stand at room temperature for at least 10 minutes. After this, 0.5to 2 mL of 0.5 M solution of TRIS was then added to the mixture, whichwas then shaken and allowed to age for 5 minutes. After aging,microscope slides (polished with cerium oxide polish, washed, andallowed to dry) were then flow coated with the final sol mixture andallowed to dry for approximately 5-10 minutes. Once dry, the slides werecured in either one of two ways. In one method, the slides were placedcoated side up on a hot plate/stirrer for 45 minutes at 120° C. In theother method, they were placed in an oven at 25° C. for 25 minutes. Thetemperature was then raised at a constant rate over a period of 20minutes to 120° C., and then maintained at 120° C. for 180 minutes. Thetemperature was then cooled to 25° C. at a constant rate over a periodof 60 minutes. With these coatings, an anti-reflective enhancement of3.45% at the maximum transmittance and 2.77% averaged over the solarrange was observed.

In one embodiment demonstrating the use of a cross-linking additivereferred to as Example 3, a sol was prepared according to the process inExample 1. After the final sonication, the mixture was allowed to chillin refrigerated water for 5 minutes and then stand at room temperaturefor at least 10 minutes. After this 1 mL of 0.5 M solution of TRIS wasadded to the mixture. After mixing the components, a variable amount (50to 250 μL) of a cross-linker was added. The sol is allowed to furtherage for 5 minutes prior to deposition on the substrate.

One variant of Example 3 includes adding a variable amount (50 μL to 250μL) of the cross-linking additive individually to each sol (Sol I andSol II) followed by the same sequence of steps. Another variant ofExample 3 includes mixing Sol I, Sol II, and TMOS along with a variableamount (50 μL to 250 μL) of the cross-linking additive followed bysonication and then adding TRIS to the mixture. Another variant ofExample 3 includes making a multilayer coating with an underlayer madefrom, for example, the coating described above in connection with theuse of TRIS as the particle size modifying additive, followed bydepositing an overlayer of 1-5% by volume of cross-linking additivedissolved in IPA.

In one embodiment demonstrating the use of a surface modificationadditive referred to as Example 4. Sol I and sol II are prepared usingthe process described in Example 1. Then equal parts (12.5 mL each) weremixed, and 100 μL of TMOS was added along with a variable amount (50 μLto 250 μL) of triethoxysilylbutyraldehyde followed by sonication for 35minutes. The sol was allowed to further age for 5 minutes prior todeposition on the substrate. The substrate containing the coating wasfurther immersed in a solution of hexamethyldisilazane (1% by weight inIPA). The coating was allowed to dry on a hot plate at 70° C. for 1 hourto allow the condensation reaction to take place between the aldehydegroup and amino groups. The substrate containing the coating was thencured at 120° C. for 45 minutes.

One variant of Example 4 includes forming the coating with the solmixture from Examples 1, 2, or 3 above followed by surface treatment ofthe coating with the silane coupling agent (1% IPA) followed bytreatment with hydrophobic reactive agent (1% IPA). Another variant ofExample 4 includes forming one of the following three sol coatings ((1)0.38% TMOS, 0.47% MTMOS, 0.56% F3TMOS, 0.05% HCl, 12.17% water, and86.37% IPA; (2) 0.38% TMOS, 0.47% MTMOS, 0.56% F3TMOS, 0.04% NH4OH,12.18% water, and 86.37% IPA; or (3) 0.34% TMOS, 0.47% MTMOS, 0.56%F3TMOS, 0.05% HCl, 12.18% water, and 86.40% IPA, where all percentagesare weight percent) followed by surface treatment with hydrophobicreactive agent (1% solution in IPA).

Where applicable, the measurement of anti-reflective properties of thecoatings was done as follows: The transmittance of the coatings wasmeasured by means of UV-vis absorption spectrophotometer equipped withan integrator accessory. The anti-reflective enhancement factor ismeasured as the relative percent increase in transmittance compared tountreated glass slides versus glass slides coated with compositions ofthis disclosure. ASTM E424 describes the solar transmission gain, whichis defined as the relative percent difference in transmission of solarradiation before and after the application of the coating. The coatingsexhibit about 1.5% to about 3.25% gain in solar transmission. Therefractive index of the coating was measured by an ellipsometer.Embodiments of the disclosure have refractive index between about 1.3and about 1.45, and between about 1.33 and about 1.42 and between about1.39 and about 1.41.

The abrasion resistance of the coating is measured by an abrader deviceaccording to European standard EN-1096-2 (glass in building coatedglass). The coatings made according to Examples 1, 2, and 3, without anyadded composition modifying additives, are able meet the passingcriteria of the standard.

The contact angle of the coatings is measured by means of goniometerwherein the contact angle of the water droplet is measured by means of aCCD camera. An average of three measurements is used for each sample.

Table 2 presents the results of several performance tests performed oncoatings made according to Example 1. In this Table, the “Spec” refersto the formal procedure for the test performed; the “Pass Criteria”refers to the allowable change in % transmittance (% T) in order for asample to pass the test; “N” is the number of samples/experimentstested; and “Results delta T” is the average of the observed change inpercent transmittance (% T) for the N samples/experiments.

TABLE 2 Pass Result Test Conditions Duration Spec# Criteria N ΔTAbrasion 400 g, 14 mm Felt Pad 1000 Strokes EN1096.2 <0.5%   — 0.2%Resistance Damp Heat 85° C./85% RH 1000 Hrs IEC61215 <3% 4 0.3% IEC61646Temperature Cycle −40° C. to +85° C. 200 Cycles IEC61215 <3% 4 0.4%IEC61646 Humidity Freeze −40° C., to +85° C., 85% 10 Cycles IEC61215 <3%4 0.2% RH IEC61646 UV Exposure New River, Arizona: 13,092 MJ/m² ASTM <3%2 0.3% EMMAQUA (Total) G90 ΔWCA<10° 3° Day spray with night 279 MJ/m²time wetting (295-385 nm) Corrosive 1 vol % of SO₂ 1200 Hrs DIN50018 <1%5 0.8% Atmosphere UL1332 Salt Spray 5% NaCl in H₂O pH 200 Hrs DIN50021<1% 5 0.5% 6.5-7.2 at 35° C. UL50 Chemical 1M HCl 30 min <0.5%   3 0.3%Resistance (Acid) 1M H₂SO₄ 3 0.4% 1M HNO₃ 3 0.3% Chemical 1M NH₄OH 30min <0.5%   3 0.1% Resistance (Base) 0.67% aq. (.1675M) 50 min 3 0.2%NaOH Boiling Water 100° C. 10 min <0.5%   4 0.0% Industrial Wet glass +toner 5 pass with squeegee <1% 3 0.2% Contaminants wash with water.Cleaning Tests Common Detergents 1000 brush <1% 3 0.3% Windex strokes 30.2%

These results are broadly similar to reliability test results achievedby existing anti-reflective coatings. However, they are significant inthat they have been achieved with a coating cured at just 120° C.Existing anti-reflective coatings are typically sintered at 400˜600° C.to achieve the level of reliability indicated by these results.

FIG. 1 illustrates the UV-vis transmittance spectra showing maximumtransmittance enhancement of 2% with coatings on glass slides made fromcomposition given in Example 1.

FIG. 2 illustrates the UV-vis transmittance spectra showing maximumtransmittance enhancement of 3.4% with coatings on a glass slidessubstrate made from composition given in Example 2.

FIG. 3a is an SEM cross-sectional view of a coating made from thecomposition of Example 1 on a glass slide substrate. The SEM images showthe absence of any discernible porosity in these coatings. The filmthickness about 90 nm.

FIG. 3b is a SEM oblique view of a coating made from the composition ofExample 1 on a glass substrate.

FIG. 4 is an SEM cross-sectional view of a coating made from thecomposition of Example 2 on a glass slide substrate. The SEM images showincreased porosity in these coatings along with presence of surfaceroughness on these coatings. The film thickness is about 100 nm.

FIG. 5 shows an XPS spectrum of a coating made from the composition ofExample 1. The peaks indicate presence of fluorine in the coatingscoming from incorporation of (3,3,3-trifluoropropyl) trimethoxysilanealong with silicon and oxygen from silicate network and carbons from theorganic components present in the coating.

FIG. 6 shows an XPS spectrum of a coating made from the composition ofExample 2. The peaks indicate presence of fluorine in the coatingscoming from incorporation of (3,3,3-trifluoropropyl) trimethoxysilanealong with silicon and oxygen from silicate network and carbons from theorganic components present in the coating.

FIGS. 7a, 7b, 8a and 8b illustrate nano-indentation data showing theindenter depth profile, hardness, and Young's Modulus of a coating madefrom the composition of Examples 1 and 2 on a glass slide substrate. Theresults of these measurements are summarized in Table 3.

Table 3 lists various properties for coatings made according to variousembodiments of the present disclosure, wherein “hardness” is amount offorce (in GPa) exerted by the indenter before the film can exhibitplastic deformations, “contact angle” is the angle of liquid/airinterface at a solid surface or the angle the liquid drop makes with asolid substrate, and “transmission increase” is calculated as {[% Tcoated−% T uncoated]/[% T uncoated]}×100. It should be appreciated thatin some embodiments, the coatings of the present disclosure provide anincrease in transmission from about 1% to about 3.5% and in someembodiments from about 1.5% to about 3%, and a contact angle of about80° to about 120° and in some embodiments about 85° to about 100°.

TABLE 3 % Average Refractive Coating Young's Transmission Contact IndexThickness Hardness Modulus Coating Increase Angle (ellipsometry)(ellipsometry) (GPa) (GPa) Example 1 1.5% to 2.2% 85-90° 1.42 70-100 nm3.98 62.55 Example 2 2.5% to 2.9% 85-95° 1.31 80-150 nm 1.64 50.60

Nano-indentation measurements were performed with a Berkovichnanoindentation system. Typical hardness for a mixture of pure silicasol-gels coatings is observed to be around 1.05 GPa. Without wishing tobe bound by theory, the enhanced mechanical properties of the coatingsof this disclosure (as compared to pure silica-based coatings) may bedue to several factors that contribute to increased hardness. First, theextensive cross-linking due to the use of the three-precursor systemmakes the Si—O—Si network stronger. Second, the combined use oforganosilane and organofluorosilane enhances the noncovalentinteractions between the organic side chains to promote betterinteractions that enhance the overall mechanical properties. Third, theincreased interactions between the side chains promote a better fillingof porous void space in the sol-gel network to make a homogenous andsubstantially nonporous coatings. Taken together, the unique combinationof precursors along with the absence of porous microstructure and theenhanced side chain interactions between the organic groups provides theimproved mechanical properties as compared to coatings of the prior art.

FIG. 9 illustrates the results of accelerated simulated soiling studiesconducted in a laboratory environment showing the change in solar photonflux passing through the slides and FIG. 10 illustrates the change inweight with respect to sequential exposure of coated and uncoated glassslides to simulated dirt storm via wind-driven deposition of dirtparticles. The anti-soiling behavior of the uncoated and coated slideswas measured using the following procedure that simulated acceleratedsoiling of objects that are placed outdoor for extended periods. Theabsorption spectra of the coated and uncoated slides were measured toestablish their transmittance. The both the slides were weighed. Theslides were then placed in a freezer at 4° C. for 5 minutes and exposedto steam from boiling water for 30 seconds to make their moist withwater vapor. These slides were then placed in dust chamber to depositdust that was evenly dispersed by blowing air from a fan. Each slide wasexposed to air dispersed with dirt for 3 minutes. The slides were thenplaced an IR lamp for 1 minute for radiant drying of the slides toremove moisture. Then the slides were placed in a wind machine that hadair from a fan to remove any loosely held dirt particles. This stepensured that only strongly bound dirt particles were retained. Theseslides were then weighed to determine the excess weight due to depositeddirt. Finally, the transmission spectra of the slides were recorded todetermine change light transmission due to deposition of dirt particles.This process was repeated for six cycles and the results of these areshown in FIGS. 9 and 10.

FIG. 11 shows a depth profile of a coating made from the composition ofExample 1 on a surface of soda-lime glass. It was obtained byalternating XPS data acquisition cycles with sputter cycles. Materialwas removed from the sample using an Ar+ source. In order to eliminatecrater wall effects, the data were acquired from a smaller region(300×1400 μm) in the center of the sputter area (4 mm×4 mm). Zalarrotation was used to minimize roughening of the samples due to ionbombardment. The sputter rate is calibrated using SiO₂, which can give arelative depth, but cannot be used as a measure of absolute depth.

FIG. 11 shows that the elemental composition of the coating isapproximately 45% O, 28% Si, 15% F and 12% C. Furthermore theconcentration of fluorine (111) in the coating is approximately constantthrough the full thickness of the coating. The disappearance of F and Cand the appearance of Ca and Na are approximately indicated by thevertical line (110) which represents the transition from the coating tothe underlying soda-lime glass substrate. By varying the ratio oforganosilane to fluorosilane the percentage concentration of fluorinewithin the coating can be varied between about 20% and 1% and furtherbetween about 15% and 5%. Without being bound by theory, as the coatingwears and abrades over time in an outdoor environment new fluorine willbe exposed at the surface. This will lead to a more persistenthydrophobic and oleophobic property when compared to typical coatingssuch as those described herein where the hydrophobic component is a thinsurface layer that quickly breaks down and wears off.

FIG. 12 shows a depth profile of a commercially available hydrophobicanti-reflective coating for solar modules manufactured by ChangzhouAlmaden Co., Ltd. It was obtained using the same method as used in thegeneration of FIG. 11. The line (120) marks the approximate transitionfrom the coating to the underlying soda-lime glass as indicated by thedisappearance of C and the appearance of Ca, Na and Mg.

FIG. 12 shows that the coating is composed of about 50% 0, 30% Si and20% C. 1% F was detected at the surface, but no F was detected in thebulk of the coating.

FIG. 13 shows an SEM oblique view and cross-section of a commerciallyavailable hydrophobic anti-reflective coating for solar modulesmanufactured by Changzhou Almaden Co., Ltd. This is another sample ofthe same coating that was analyzed in FIG. 12. It can be observed thatthe coating is approximately 100 nm to 150 nm thick. On the top surfaceof the coating, thin, circular disc-like structures are observed. Eachdisc has an associated raised bump. The sample exhibited a water contactangle of approximately 85° when measured by goniometer and is marketedas hydrophobic. The presence of these disc structures is consistent withthe theory that a hydrophobic material, possibly a fluorosilane, hasbeen deposited by spray or otherwise onto the finished surface of thecoating to impart the hydrophobic property.

Commercially available hydrophobic glass coatings such as the ChangzhouAlmaden coating analyzed here are formed by applying a thin continuousor discontinuous mono-layer of fluorosilane to the surface. Thesecoatings do not exhibit persistent hydrophobicity when exposed to anoutdoor environment. The thin layer of fluorocarbon attached to thesurface is prone to removal by abrasion during cleaning and fromenvironmental factors such as windborne dust. It is also prone tooxidative breakdown caused by exposure to UV radiation and to moisture.In general, these classes of coatings do not last more than 12˜24 monthsin typical outdoor environments and may last for much shorter times inharsh environments such as desert or tropical areas, such as for examplein Arizona and Florida.

FIG. 14 shows results of surface and bulk elemental analysis using XPSon two samples prepared according to the method of Example 1 that havebeen aged over 3000 hours. Sample 1 used a ratio of organosilane tofluorosilane of 1 and Sample 2 used a ratio of 0.5. A Ci4000Weather-Ometer (ATLAS Material Testing Technology LLC) was used toperform indoor aging of the samples. The chamber temperature of 60° C.and relative humidity (RH) of 60% were used in the instrument. Waterspray was not used during the aging. Specimens were placed on a carouselthat rotates about a xenon-arc lamp. An inner glass filter (Right Light,ATLAS Material Testing Technology LLC) was used to filter the lamp inconjunction with a coated infrared absorbing quartz outer glass filterso that the lamp closely replicates the AM1.5 spectrum, with the powerlevel being controlled to 114 W·m⁻² for 300≦λ≦400 nm, i.e., 2.5× theAM1.5 global spectrum in IEC 60904-3. The Ci4000 therefore provides ˜10×the raw UV daily dose, based on its intensity and continuous operationthroughout the day. The duration of 3000 cumulative hours wouldtherefore be equivalent to at least 3.5 years in the field, based on UVirradiation alone. For aging in the Ci4000, specimens were placed instainless steel holders (ATLAS Material Testing Technology LLC), withthe coated surface facing the Xe lamp. Specimens were not masked in theCi4000, other than at the 5 mm periphery of the sample holders used tosecure the specimens within the Ci4000.

The samples were analyzed with XPS before the aging was started and thenat the 1000, 2000 and 3000 hour read-points. At all read-points the XPSanalysis was carried out on the coating surface. At the 2000 and 3000hour read-points, the XPS analysis was also carried out after 20 secondsand 60 seconds of Ar+ sputtering. This step etches away some of thecoating material to expose fresh underlying material. It also removesany surface contamination. In the case of fluorine concentration, therewas good correlation between the surface concentration and the interiorconcentration, suggesting there was little external contamination of thesurface by fluorine.

FIG. 14 shows that the fluorine content of the coating that isresponsible for the persistent hydrophobic property is robust whenexposed to accelerated environmental conditions of UV, temperature andhumidity. While the fluorine content is reduced over the period of thestress, more than half remains at the end of the stress. Additionally,the rate of reduction of fluorine as evidenced by the slope of the graphbetween the 2000 hour and 3000 hour read-points relative to the slope ofthe graph between the 0 hour and 2000 hour read-points is less.

Embodiments described herein are well suited to performing various othersteps or variations of the steps recited herein, and in a sequence otherthan that depicted and/or described herein.

It should be appreciated that reference throughout this specification to“one embodiment” or “an embodiment” means that a particular feature,structure or characteristic described in connection with the embodimentis included in at least one embodiment of the present disclosure.Therefore, it is emphasized and should be appreciated that two or morereferences to “an embodiment” or “one embodiment” or “an alternativeembodiment” in various portions of this specification are notnecessarily all referring to the same embodiment. Furthermore, theparticular features, structures or characteristics may be combined assuitable in one or more embodiments of the disclosure.

Similarly, it should be appreciated that in the foregoing description ofexemplary embodiments of the disclosure, various features of thedisclosure are sometimes grouped together in a single embodiment,figure, or description thereof for the purpose of streamlining thedisclosure aiding in the understanding of one or more of the variousinventive aspects. This method of disclosure, however, is not to beinterpreted as reflecting an intention that the claimed disclosurerequires more features than are expressly recited in each claim. Rather,as the following claims reflect, inventive aspects lie in less than allfeatures of a single foregoing disclosed embodiment. Thus, the claimsfollowing the detailed description are hereby expressly incorporatedinto this detailed description, with each claim standing on its own as aseparate embodiment of this disclosure.

While the disclosure has been disclosed in connection with the preferredembodiments shown and described in detail, various modifications andimprovements thereon will become readily apparent to those skilled inthe art. Accordingly, the spirit and scope of the present disclosure isnot to be limited by the foregoing examples, but is to be understood inthe broadest sense allowable by law.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the disclosure (especially in the context of thefollowing claims) is to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. Recitation of ranges of valuesherein are merely intended to serve as a shorthand method of referringindividually to each separate value falling within the range, unlessotherwise indicated herein, and each separate value is incorporated intothe specification as if it were individually recited herein. All methodsdescribed herein can be performed in any suitable order unless otherwiseindicated herein or otherwise clearly contradicted by context. The useof any and all examples, or exemplary language (e.g., “such as”)provided herein, is intended merely to better illuminate the disclosureand does not pose a limitation on the scope of the disclosure unlessotherwise claimed. No language in the specification should be construedas indicating any non-claimed element as essential to the practice ofthe disclosure.

While the foregoing written description enables one of ordinary skill tomake and use what is considered presently to be the best mode thereof,those of ordinary skill will understand and appreciate the existence ofvariations, combinations, and equivalents of the specific embodiment,method, and examples herein. The disclosure should therefore not belimited by the above described embodiment, method, and examples, but byall embodiments and methods within the scope and spirit of thedisclosure.

All documents referenced herein are hereby incorporated by reference.

1. A method of forming a glass coating, comprising: making a sol byhydrolyzing an organosilane in the presence of a least one solvent andat least one catalyst; further adding at least one alkoxysilane; andaging the sol for at least 24 hours.
 2. The method of claim 1, whereinthe organosilane comprises methyltrimethoxysilane.
 3. The method ofclaim 1, wherein the alkyoxysilane comprises at least one oftetramethoxysilane and tetraethoxysilane.
 4. The method of claim 1,wherein the at least one solvent comprises at least one member selectedfrom the group consisting of water, methanol, ethanol, 1-propanol,2-propanol, n-butanol, isobutanol, tert-butanol, pentanol, hexanol,cyclohexanol, ethylene glycol, propylene glycol, ethylene glycolmonomethyl ether, ethylene glycol dimethyl ether, propylene glycolmonomethyl ether, ethylene glycol dimethyl ether, propylene glycolmonomethyl ether, diglyme, acetone, methyl ethyl ketone (butanone),pantanones, methyl t-butyl ether (MTBE), ethyl t-butyl ether (ETBE),acetaldehyde, propionaldehyde, ethyl acetate, methyl acetate, and ethyllactate.
 5. The method of claim 1, wherein at least one catalystcomprises at least one member selected from the group consisting ofhydrochloric acid, nitric acid, acetic acid, trifluoromethanesulphonicacid, oxalic acid, sodium hydroxide, aqueous ammonia, pyridine,tetraethylammoniuum hydroxide, and benzyltriethylammonium hydroxide. 6.The method of claim 1, wherein the sol is aged for between 24 and 48hours.
 7. The method of claim 1 further comprising: a. preparing acoated glass substrate, wherein the preparing comprises applying theaged sol to a glass substrate; b. drying the sol; and c. curing the solat a temperature of between 20° C. and 200° C., wherein the coated glasssubstrate has at least one of an anti-reflective property, a highabrasion resistance property, a hydrophobic property, and ananti-soiling property.
 8. The method of claim 7, wherein the coatedglass substrate is further assembled into a solar module.
 9. The methodof claim 7, wherein the coated glass substrate comprises float-glass,the method further comprising applying the sol on a tin-side of theglass substrate.
 10. The method of claim 7, wherein the glass substratecomprises tempered glass.
 11. The method of claim 7, wherein applyingthe aged sol further comprises applying by a technique selected from thegroup consisting of roll coating, dip coating, spraying, drop rollingand flow coating.
 12. The method of claim 7, wherein the curingcomprises an operation to cure the sol under ambient conditions.
 13. Themethod of claim 7, wherein the curing comprises an operation to heat thesol to 120° C.
 14. A method, comprising: preparing a sol by hydrolyzingan organosilane in the presence of a least one solvent and at least onecatalyst, the catalyst comprising at least one of an acid catalyst and abase catalyst; further adding at least one alkoxysilane; and aging thesol for at least 30 minutes.
 15. The method of claim 14, furthercomprising aging the sol for a period between 24 and 48 hours.
 16. Themethod of claim 15, wherein the at least one catalyst comprises at leastone acid catalyst, wherein the at least one acid catalyst comprises atleast one member selected from the group consisting of hydrochloricacid, nitric acid, acetic acid, trifluoromethanesulphonic acid, andoxalic acid.
 17. The method of claim 15, wherein the at least onecatalyst comprises at least one base catalyst, wherein the at least onebase catalyst comprises at least one member selected from the groupconsisting of sodium hydroxide, aqueous ammonia, pyridine,tetraethylammoniuum hydroxide, and benzyltriethylammonium hydroxide. 18.The method of claim 16, further comprising: preparing a coated glasssubstrate, wherein the preparing comprises applying the aged sol to aglass substrate; drying the sol; and curing the sol at a temperature ofbetween 20° C. and 200° C., wherein the coated glass substrate has atleast one of an anti-reflective property, a high abrasion resistanceproperty, a hydrophobic property, and an anti-soiling property.
 19. Themethod of claim 18, wherein the glass substrate comprises float-glass,the method further comprising applying the sol on a tin-side of theglass substrate.
 20. The method of claim 19, wherein the curingcomprises an operation to heat the sol to 120° C.